A Flexible Containment Mechanism for Executing Untrusted Code

David S. Peterson, Matt Bishop, and Raju Pandey
Department of Computer Science
University of California, Davis
{peterson, bishop, pandey}@cs.ucdavis.edu

Abstract

1 Introduction

The limitations of the UNIX security model have created much interest in alternate paradigms. This has drawn attention to a wide variety of mechanisms. Examples are capabilities[1], access control lists (ACLs), domain and type enforcement (DTE)[2, 3], and sandboxing mechanisms. Sandboxes are attractive because they provide a centralized means of creating security policies tailored to individual programs and confining the programs so that the policies are enforced. They therefore provide great potential for simplifying system administration, preventing exploitation of security holes in system programs, and safely executing potentially malicious code. Their value as security tools increases as computing environments become more network-centered and execution of downloaded code becomes more common.

A number of methods have been proposed for confining untrusted programs. Although these techniques have individual strengths, they also have limitations that narrow the scope of their applicability. In this paper, we systematically explore the range of options available to designers of sandboxing mechanisms. As we discuss various design choices and their consequences, we present a sandboxing facility that combines the advantages of a number of alternatives. Our sandboxing mechanism is implemented as a system call API that serves as a general-purpose framework for confining untrusted programs. Our goal is to provide primitives that are simple yet powerful enough that system administrators, individual users, and application developers may use them to specify and enforce security policies that are custom-tailored to satisfy their diverse needs.

In the next section, we present the design of our sandboxing facility within the context of various design alternatives and the motivations behind them. Section 3 provides details of how privileges are represented in our design. In Section 4, we give preliminary performance results from a partially completed implementation within the Linux kernel. Section 5 contains an overview of related works and how they differ from our design. Finally, we present conclusions in Section 6.

2 Design Alternatives

1. Sandboxes may grant or deny various privileges to the programs that they contain. How are these privileges represented and organized?
   
   
2. Where are the mechanisms located that enforce sandbox-imposed restrictions?
   
   
3. Are restrictions enforced by passive or active entities¹?
   
   
4. Are sandboxes global entities that enforce systemwide constraints or more localized entities that confine individual programs or perhaps groups of related programs? What criteria are used to group programs into sandboxes?
   
   
5. Do sandboxes enforce mandatory or discretionary access controls?
   
   
6. How are access privileges determined for inspection and manipulation of sandbox configurations?
   
   
7. Are sandboxes static or dynamic entities? In other words, are their configurations fixed or subject to change? If sandboxes are reconfigured in response to changing security policies, how do the changes propagate throughout a running system?
   
   
8. Are sandboxes generic entities for entire classes of programs, or are they narrowly customized for specific programs?
   
   
9. Are sandboxes transient or persistent entities? Do they function as lightweight, disposable containers, or do they maintain relatively static long-term associations with programs and other objects that they may contain?
   
   
10. How do sandboxes interact with other security mechanisms?
    
    

We have developed a kernel-based mechanism that provides a general-purpose system call API for confining untrusted programs. Processes may create their own sandboxes, launch arbitrary programs inside them, and dynamically reconfigure the sandboxes as programs execute inside. Unprivileged processes may safely create and configure sandboxes because our mechanism follows the principle of attenuation of privileges. Specifically, a sandbox can never grant privileges to a program beyond what the program would normally have if it were not executing inside the sandbox. Consider the following example of how our facility might typically be used:

1. A process creates a new sandbox by making an sbxcreate() system call. The newly created sandbox is assigned a numeric identifier that is conceptually similar to a filename. The creator receives a numeric handle that is essentially the same as a file descriptor. Initially, only the creator can access the sandbox.
   
   
2. The process configures the sandbox using additional system calls.
   
   
3. The process forks and the child inherits a copy of the parent's sandbox descriptor.
   
   
4. The child applies the sandbox to itself by making an sbxapply() system call. This can be done in one of two ways:
   
   
   1. No options are specified when calling sbxapply(). On return, the sandbox is applied to the child. The apply operation automatically closes any sandbox descriptors held by the child. The child therefore gives up control of all sandboxes it formerly controlled, including the one that now contains it.
      
      
   2. The "apply on exec" option is passed to sbxapply(). The child then performs an execve() system call. If execve() succeeds, the sandbox is applied to the child and all of its sandbox descriptors are closed. On failure, the sandbox is not applied. Thus the child retains any privileges necessary for error handling.
      
      
   
   
   
5. The parent retains full control over the sandbox and may reconfigure it while the child executes inside. The parent may also launch additional programs inside the sandbox. Alternately, it may close its sandbox descriptor, giving up all access rights and eliminating itself as a potential point of attack. The sandbox is now unchangeable by any process, even those with root privileges. Although the child is trapped in the sandbox for the rest of its lifetime, outside processes can still suspend or terminate it. Sandboxes only impose restrictions on the processes they contain. They never place limits on what outside processes can do relative to processes executing within.
   
   
6. All of the child's descendants inherit its sandbox. A process may be sandboxed only by applying a sandbox to itself or inheriting its parent's sandbox.
   
   
7. There is no explicit destroy operation for sandboxes. The kernel manages their destruction through reference counting.
   
   

2.1 Representation and Organization of Privileges

As computer systems evolve to serve new purposes, new features are added to operating systems. A sandboxing mechanism should therefore be easy to extend so that it may enforce security policies governing access to new types of system resources. With this requirement in mind, we have divided system functionality into several categories, each represented by a different component type. As new features are added to operating systems, our mechanism may be extended by creating additional component types. To facilitate their development, we have structured our implementation in a modular fashion. Our current design specifies the following seven types of components:

• Device component: Specifies access privileges for devices according to device number.
  
  
• File system component: Specifies access privileges for files according to directory path.
  
  
• IPC component: Specifies access privileges for IPC objects such as semaphores, message queues, and shared memory segments.
  
  
• Network component: Specifies ranges of IP addresses to which sandboxed processes may open connections. Also specifies ranges of ports from which incoming connections may be received.
  
  
• ptrace() component: Specifies which processes a sandboxed process may ptrace().
  
  
• Signal component: Specifies processes to which a sandboxed process may send signals.
  
  
• System management component: Specifies privileges for administrative actions such as rebooting and setting system date/time.
  
  

To permit flexible specification of fine-grained security policies, privileges must be specified in a highly expressive manner. With this goal in mind, we divide privileges into two categories: binary privileges and quantitative privileges. A binary privilege may be assigned one of two possible values: allow or deny. An example is the ability to read the contents of /etc/passwd. A quantitative privilege may be assigned numeric values such as 50 or 100. For example, the total memory allocated to a program might be restricted to a maximum of 4 megabytes.

Our current design only deals with binary privileges. Quantitative privileges address issues regarding denial of service. The addition of features that guard against these types of attacks is an area of future work. We intend to study solutions that others have developed[5, 6] and incorporate them into our design.

The two possible values of a binary privilege may be viewed as membership in or exclusion from a set of allowed operations. This insight suggests the following approach: Represent sets of privileges as first-class objects and provide primitives for manipulating them using set-theoretic transformations. Our components are designed to behave in exactly this manner. Specifically, given two components x and y of a given type, we provide the following operations:

• Create union: Create a new component z that represents the union of the privileges given by x and y.
  
  
• Create intersection: Create a new component z that represents the intersection of the privileges given by x and y.
  
  
• Create complement: Create a new component z that represents the complement of the privileges given by x.
  
  
• Union with self: Modify x so that it represents the union of y with its prior value.
  
  
• Intersect with self: Modify x so that it represents the intersection of y with its prior value.
  
  
• Complement self: Modify x so that it represents the complement of its prior value.
  
  

B := (B intersect not(P)) union F

B := B union (G intersect not(G_C))

• Set operations are very expressive. They allow components to be constructed that satisfy assertions relative to each other given by arbitrary set-theoretic expressions.
  
  
• Set theory is well-understood. Therefore, so are relationships among components.
  
  
• Set operations provide a means of manipulating privileges that is uniform across all component types. This exemplifies the principle of economy of mechanism presented by Saltzer and Schroeder[4] and is likely to simplify programs that use our sandboxing API.
  
  
• Set operations provide a means of answering questions such as "Which privileges are granted to user A or user B but denied to user C?" This information may be useful if we wish to know how much damage user C can inflict if he successfully bribes users A and B. In general, a convenient means of answering such questions allows one to easily understand implications of various sandbox configurations.
  
  
• By clarifying relationships between sandbox-associated privileges, set operations provide a means of verifying that security policies are correctly enforced.
  
  
• Providing users with simple yet powerful mechanisms often results in the development of new and useful applications.
  
  

2.2 Location of Enforcement Mechanisms

• runtime environment
  
  
• sandboxed program
  
  
• user space³
  
  
• OS kernel
  
  

2.2.1 Runtime Environment

2.2.2 Sandboxed Program

2.2.3 User Space

2.2.4 OS Kernel

We have chosen to implement our sandboxing mechanism within the OS kernel. The kernel-resident status of our implementation allows us to export a universally accessible system call API that may be applied to both privileged and unprivileged programs, regardless of what language they were written in. Our system call API is designed to be policy-neutral and highly flexible. It provides a minimal set of primitives that are designed to serve a wide variety of purposes. Thus, application-dependent aspects of sandbox manipulation are pushed into user space where they belong. The general-purpose nature of our design mitigates the disadvantages of kernel code being difficult to develop and debug.

2.3 Passive vs. Active Monitoring

To address this design issue, we have developed a novel mechanism that allows monitoring to be purely passive, purely active, or anywhere in between. Thus, programs may benefit from the best aspects of both alternatives. We achieve these benefits through a mechanism that allows privileges to be determined interactively at runtime. Specifically, a sandbox may be configured so that attempting certain actions will cause a sandboxed process to block instead of being immediately denied the privilege to perform the action. When a process blocks in this manner, an event is generated and placed in the event queue of the sandbox where the blocking occurred. A process that has ownership over the sandbox uses the sbxwait() system call to wait for and obtain events. An event may be examined to determine which process generated it and what action was attempted. The sbxdecide() system call is then used to unblock the process that triggered the event and decide whether to allow the attempted action.

Figure 1: Blocking mechanism

Our design permits application of the blocking mechanism in a fine-grained manner. Figure 1 illustrates how this works. Each sandbox has two sets of attachment points for the various component types. Sandbox S has device components D₁ and D₂ attached at points d₁ and d₂. File system component F₁ is attached at point f₁. Process p controls sandbox S while q executes inside. When q attempts to access a device, the sandboxing mechanism first examines D₁. If D₁ allows the required privilege then the operation will succeed⁴. Otherwise, D₂ is examined. If D₂ allows the privilege, then q blocks and p decides whether to allow the operation. If D₂ denies the required privilege, then the operation will fail. If q attempts to access some file, the sandboxing mechanism examines component F₁. If F₁ allows the required privilege, then the operation is allowed. Otherwise, the operation is immediately denied, since no component is attached at point f₂.

A potential use of this feature is intrusion detection. For example, a telnet daemon could place a user's login shell inside a sandbox and use the blocking feature to monitor aberrant behavior. If such behavior is detected, the system can make fine-grained adjustments to the set of actions that it monitors. In response to suspicious behavior, the system may tighten sandbox-imposed constraints, or perhaps perform other actions such as notifying a system administrator.

2.4 Scope of Application: Global vs. Local

Figure 2: Nested sandboxes

We believe that there is no single best answer to these questions. Therefore our design allows system administrators, users, and application developers to create sandboxes that enforce security policies at any level of granularity. To permit simultaneous enforcement of access controls at multiple levels, our design provides the ability to create hierarchically nested sandboxes, as shown in Figure 2.

In this example, sandbox G is a global sandbox that contains all processes. G enforces global policies such as the restriction that no process should be able to modify system programs in locations such as /bin and /usr/bin. At system startup time, /sbin/init creates G and applies G to itself before it forks any child processes. To override the restrictions imposed by G, an administrator with physical access to the system console must reboot the system with a kernel in which sandboxing functionality has been disabled.

At a more localized level, programs such as telnet daemons, ftp daemons, and the standard login program may be modified to place restrictions on individual users. Sandboxes A and B restrict the login shells of users Alice and Bob in this manner.

Users may selectively delegate their privileges by creating sandboxes for individual applications. For instance, user Alice has downloaded a video game from an untrusted source. To protect against Trojan horses, she executes the program inside sandbox X.

Finally, an application program that is aware of the sandboxing mechanism may use it as a flexible means of dropping privileges when performing sensitive operations. The web server executing in sandbox W uses our mechanism in this manner by executing CGI programs in sandboxes C₁ and C₂.

If the blocking mechanism is used in combination with nested sandboxes, an attempted action by a sandboxed process may cause it to block sequentially at multiple levels. For instance, if the downloaded game in sandbox X attempts to open some file, the privilege checking operation performed at sandbox X may cause it to block. If a process in sandbox A decides to allow the action, then a privilege check will be performed at sandbox A. Depending on how A is configured, this may also cause the process to block, providing an opportunity for a process in sandbox G to allow or deny the action. The same behavior could also take place at sandbox G if it were configured appropriately, although this would require some process outside G to be responsible for monitoring G. In practice, we believe that sandboxes will rarely be nested at depths of more than three or four levels. Therefore the overhead required to perform privilege checks at multiple levels should be reasonably low.

2.5 Mandatory vs. Discretionary

Our design provides both options. One means of providing mandatory access controls is to place /sbin/init in a sandbox at system startup time. Additionally, sandboxes may enforce mandatory access controls at the level of individual users. Since our mechanism follows the principle of attenuation of privileges, unprivileged users may employ it to create discretionary sandboxes.

As future work, we intend to add a mechanism that allows transitions between sandboxes when certain programs are executed. This would make sandboxes more similar to the domains provided by DTE[2, 3]. However, the use of components to define privileges granted to domains is a different approach from using types. Using our mechanism, a core set of components may be defined that serve the same purpose as types. Additional types can be derived using set-theoretic transformations. Permitting dynamic creation of types at runtime may also be useful. For instance, executing a certain program might cause creation of a new type that is a function of the user's previous type and possibly other variables.

2.6 Inspection and Manipulation of Sandboxes

The question of how access to sandboxes should be governed is open-ended and depends on the details of the mechanism being considered. We have taken a conservative approach in which access is strictly limited. A descriptor with read privilege is required for examining the configuration of an object. Likewise, a descriptor with write privilege is required for calling sbxwait() on a sandbox or modifying an object. Descriptors may be obtained only as follows:

• The creator of an object receives a descriptor with both read and write privileges for the new object.
  
  
• When a process forks, the child inherits all of the parent's descriptors along with their associated privileges.
  
  
• If a process inside a sandbox creates an object, it may specify that a link is created for the new object. Other processes in the same sandbox may then use the sbxopen() system call to open descriptors for the new object. This is analogous to accessing files with the open() system call. Processes inside a given sandbox may therefore have shared access to child objects.
  
  
• There is only one circumstance in which processes not within the immediate boundaries of a given sandbox may open descriptors for its child objects. When creating a component, a process may label it as public. In this case, processes in descendant sandboxes may open descriptors for the component with read-only access.
  
  

2.7 Static vs. Dynamic

When adjustments to security mechanisms are made, they should ideally have an immediate effect on all relevant aspects of system behavior. Our implementation of nested sandboxes was designed with this consideration in mind. Since privilege checks are done individually at each level, reconfiguration of a sandbox immediately effects all of its descendants.

File descriptors represent a similar area of concern. For instance, suppose that a process opens some file and its sandbox is then adjusted so that access to the file is denied. Under our current implementation, the process may continue to access the file through its previously opened file descriptor. Adding the ability to revoke privileges stored in file descriptors would be relatively easy. This may be done by attaching sandbox-related tags to file descriptors and performing additional privilege checks during read() and write() system calls. Although this option has little value for guarding confidentiality, it may still be useful as a damage control mechanism for protecting data integrity. We may therefore eventually implement this feature.

2.8 Generic vs. Specific

To address these problems, one may create generic protection domains for groups of programs with similar behavior. A sandboxing mechanism known as MAPbox[11] employs this technique. Although this approach may simplify sandbox construction, appropriate behavior classes may be difficult to create. If privileges are defined too conservatively, then the scope of applicability of each behavior class becomes unacceptably narrow. However, loosely specified behavior classes stray from the principle of least privilege. Some application-specific differences among programs within a behavior class may be handled by a technique that MAPbox refers to as parameterization. For instance, a group of network-oriented services may function in a similar manner but differ in the ports from which they receive incoming connections. In this case, their behavior class may take a port number as a parameter.

Using our facility, behavior classes could potentially be represented as groups of components. Set operations could then be employed to create customized versions for individual programs in a manner somewhat similar to parameterization.

Alternately, our blocking mechanism may be used to create custom-tailored sandboxes for individual applications. For example, consider the following sequence of events:

1. A user executes a program inside a sandbox. The user has no way of knowing ahead of time what privileges it will require. Therefore the sandbox is made initially very restrictive.
   
   
2. When the program attempts to perform a denied action, it blocks and the user learns exactly what happened. The user can then decide to allow or deny the action. To allow all future operations of this type, the user may adjust the appropriate component.
   
   
3. When the sandboxed program terminates, the user may save the final sandbox configuration to be reused when executing the program in the future.
   
   

2.9 Transient vs. Persistent

WindowBox[13], a sandboxing system implemented within the Windows NT kernel, is a design in which sandboxes are persistent entities. It consists of a set of desktops that are completely separate from each other and from the rest of the system. Users may give some desktops more privileges than others. They may also place individual programs and other files within a given desktop. The association between a file and its desktop persists until the user either deletes the file or moves it to a different desktop. This feature is useful because a given program is automatically confined to its desktop whenever the user executes it. Therefore, the security policy associated with the desktop is consistently enforced. Associations between files and their desktops also provide an alternate means of defining privileges. Specifically, access may be granted because a file resides in the same desktop as the program attempting to open it.

A potential advantage of defining sandboxes as transient entities is that they may be efficiently discarded when no longer needed. Our design provides a feature that eliminates unnecessary overhead for creating and destroying sandboxes. With this option, a server may create pools of sandboxes for different types of client connections. The server does the following for each client connection:

1. The server forks a child process. The child inherits the parent's descriptors for the various sandbox pools that the server created.
   
   
2. The child makes an sbxapply() system call, passing in a descriptor for the appropriate pool. If the pool is not empty, this causes a sandbox to be removed from the pool. Otherwise, a new sandbox is created and associated with the pool. The newly obtained sandbox is applied to the child, which then handles the client request.
   
   
3. When the child dies, the reference count on its sandbox drops to zero. Instead of being destroyed, the sandbox is returned to the pool for later reuse.
   
   

2.10 Interaction with Other Security Mechanisms

1. Can other mechanisms override the denial of a privilege by a sandbox?
   
   
2. If a sandbox grants a given privilege, can other mechanisms override this decision?
   
   

3 Specification of Privileges

• Interval lists allow specification of intervals of values over a fixed range. For instance, we could use an interval list to represent all integers between 10 and 100, the value 250, and all integers between 400 and 500. The components use this data structure in several places.
  
  
• Sandbox sets specify privileges that allow sandboxed processes to perform actions relative to other processes. The ability to send signals is an example of this type of privilege.
  
  

3.1 Interval Lists

Figure 3: Include operation		Figure 4: Exclude operation

Interval lists provide a convenient way of specifying and manipulating sets of unsigned integers. They support the following operations:

• Include: Figure 3 illustrates the include operation. In this example, an interval list initially specifies the intervals { (3, 7), (10, 15) }. The interval (8, 12) is then included. This produces the interval list { (3, 15) }. Notice that this result is obtained rather than { (3, 7), (8, 15) } or { (3, 7), (8, 12), (10, 15) }. Interval lists always merge intervals together so that no two intervals are overlapping or immediately adjacent to each other. This yields the simplest possible representation.
  
  
• Exclude: Figure 4 illustrates the exclude operation. In this example, we start with the interval list { (5, 7), (9, 9), (11, 15) }. The interval (6, 12) is then excluded. This produces the interval list { (5, 5), (13, 15) }.
  
  
• Intersection: This operation takes two interval lists as operands and produces a new interval list representing the intersection of the sets of integers they specify. The intervals contained in the result are all nonoverlapping and separated by at least one integer value.
  
  
• Union: This operation is similar to intersection, except that the union is computed.
  
  
• Complement: This operation takes an interval list and produces its complement. For instance, the complement of { (5, 10) } is { (0, 4), (11, UINT_MAX) }.
  
  
• Query point: This operation takes an integer as a parameter and returns a Boolean value indicating whether any interval in the list contains it.
  
  

3.2 Sandbox Sets

Figure 5: Behavior of sandbox sets

Figure 5 illustrates how sandbox sets operate. Signal components C₁, C₂, and C₃ are attached to sandboxes S₁, S₂, and S₅ respectively. C₁ allows S₁ to signal processes in S₉, C₂ allows S₂ to signal processes in S₃ and S₉, and C₃ allows S₅ to signal processes in S₆ and S₇. Process p created and initialized S₁, S₉, and C₁. The following rules govern the behavior of components implemented using sandbox sets:

• A process in a given sandbox is always allowed to access other processes in its own sandbox or any descendant sandboxes. For example, a process in S₂ may signal any process in S₂, S₅, S₆, or S₇ regardless of how C₂ is configured.
  
  
• If a component grants access to a given sandbox, then access is also granted to all of the sandbox's descendants. For instance, processes in S₁ can signal processes in S₁₀ since C₁ grants access to S₉ and S₁₀ is a descendant of S₉. The motivation for this behavior may be understood by considering the viewpoint of process p. Clearly, p is aware of the existence of S₉. However, p can not in general be expected to keep track of actions, such as creating child sandboxes, that may be performed by processes in S₉. All p cares about is that processes in S₁ are granted access to all processes that S₉ governs. Thus this rule allows processes to manipulate components without needing to be aware of details that are outside their scope of concern.
  
  
• A process in a given sandbox may delegate to child sandboxes any access rights to other sandboxes that it possesses. For example, S₁ has adjusted C₂ so that its privilege for signaling processes in S₉ is passed down to S₂. Similarly, S₂ may adjust C₃ so that processes in S₅ can signal processes in S₉. However, S₂ may not adjust C₃ so that processes in S₅ are granted access to S₁, S₂, or S₄. This is because S₂ does not have access to S₁, S₂, or S₄. In general, any sandboxes in shaded area X could potentially appear in C₁. However, C₁ can not specify S₁₀ directly because S₁₀ is outside C₁'s scope of concern. Likewise, any sandboxes in shaded areas X or Y but not Z could potentially appear in C₂.
  
  
• All processes that are not in any sandbox are grouped together as if they are all inside a common sandbox that imposes no restrictions. This can be thought of as the "null sandbox", and may be specified in a sandbox set just like any other sandbox.
  
  
• It is possible to compute the complement of a sandbox set. For instance, the complement of the set given by C₃ would be a set that grants access to all sandboxes (including the null sandbox) except S₆ and S₇. Likewise, intersections and unions of sandbox sets may be computed.
  
  

3.3 Signal, ptrace(), and IPC Components

3.4 File System Component

• r: For a normal file, this privilege allows the file to be opened for reading. For a directory, it allows the directory contents to be listed.
  
  
• w: For a normal file, this privilege allows the file to be opened for writing. For a directory, it allows files in the directory to be created, unlinked, or renamed.
  
  
• x: For a normal file, this privilege allows the file to be executed. For a directory, this privilege has no meaning.
  
  
• p: For both normal files and directories, this privilege allows permission-related settings to be changed. Specifically, it allows use of chmod(), chown(), and chgrp().
  
  
• t: For both normal files and directories, this privilege allows changing access and modification times using utime().
  
  
• s: For a directory, this privilege allows opening files in the directory, accessing subdirectories, and moving into the directory using chdir(). For a normal file, this privilege has no meaning.
  
  

Figure 6: Directory subtree

For each of these privileges, a set of three labels is attached to each node. Figure 6 illustrates the meanings of the labels. Set S consists of the entire subtree rooted at directory n₁. Set T consists of n₁ and all of its children. Set U consists only of n₁. Given these definitions, the three labels attached to n₁ for a given privilege are defined as follows:

• self: This label represents set U (consisting of only n₁).
  
  
• children: This label represents the set of nodes defined by T - U (n₂ and n₃ in the figure).
  
  
• grandchild subtrees: This label represents the set of nodes defined by S - T (n₄, n₅, n₆, and all of their descendants).
  
  

• A label at a node has higher precedence than labels at any of its ancestors.
  
  
• There is no ordering among the three labels at a node. This is because the labels represent disjoint sets of nodes.
  
  

Figure 7: File system component

Figure 7 illustrates a file system component. It shows labels only for the w privilege. Labels for the other five privileges have been omitted for simplicity. Given the above rules, this file system component is interpreted as follows:

• Write access to the root directory is allowed, since its self label has a value of allow.
  
  
• Write access is denied for all files in the root directory except /a. Since the children label of the root directory is unspecified, it takes on the default value of deny that denies all file-related privileges for an empty tree.
  
  
• Write access is also denied to /a. Since its self label and the root directory's children label are both unspecified, it takes on the default value of deny that denies all file-related privileges for an empty tree.
  
  
• For all files in /a except /a/b, write access is denied. This is due to the setting of the children label for /a.
  
  
• Write access is allowed for the file /a/b, since its self label has a value of allow.
  
  
• Write access is allowed for all descendants of /a/b. This is because the grandchild subtrees label of the root directory is not overridden by any labels with higher precedence that affect descendants of /a/b.
  
  

3.5 Network Component

3.6 Device Component

3.7 System Management Component

4 Performance

Our implementation involves modifying fork(), execve(), exit(), and wait(). We have therefore measured the amount of overhead that our mechanism adds to each of these system calls. All experiments were performed on a uniprocessor 266 MHz Pentium II PC with 96 Mb of memory. The Linux kernel we used is an SMP build of version 2.4.1. Each value in Table 1 represents the mean value from 10000 separate system call invocations. As shown, our modifications typically add several microseconds to each call.

During a fork(), sandbox-related state information must be copied from the parent process to the child. On execve(), a check is performed to see if a sandbox must be applied due to a previous invocation of sbxapply() with the "apply on exec" option specified. The values in Table 1 reflect the typical case in which no sandbox is applied. We measured separately the latency of an sbxapply() system call (without "apply on exec" specified) and found that value to be 56 microseconds.

During an exit() system call, our implementation closes any open descriptors for sandboxes and components. It then releases the reference to any sandbox the process may be executing within and does a partial cleanup of the sandbox if the reference count drops to 0. Additional cleanup of sandbox-related state is performed during wait() when the zombie process is collected. At this time, the expired sandbox is queued so that a kernel thread may perform the final cleanup. The values for exit() and wait() in Table 1 represent the case in which this cleanup activity occurs for a single expired sandbox. The purpose of the kernel thread is to remove the sandbox from the global matrix described in Section 3.2 and free the memory that it occupies. The thread is awakened periodically when the number of expired objects on its queue reaches a certain threshold. It then deletes all of them in a single operation. We measured the time required to delete 1024 expired sandboxes, and found that this operation takes 2829 microseconds (2.8 microseconds per sandbox). This represents the mean for 10 separate invocations of the kernel thread. Adding the per-sandbox value to the overhead values in Table 1 for exit() and wait() provides a rough idea of the total overhead required for destroying a sandbox.

Figure 8: Latency of kill() executed by sandboxed process

Additionally, we measured the latency of the kill() system call when executed by a sandboxed process. The results are shown in Figure 8. For this experiment, we configured the sandbox of the sending process p so that its sandbox allows sending signals to the receiving process q, which has been placed within a separate sandbox. The values represent latencies when p is placed in sandboxes nested at various depths. For instance, the value 3 on the horizontal axis represents the case in which the sandbox enclosing p has a parent and a grandparent. Therefore, privilege checks occur at three separate levels. The value 0 on the horizontal axis indicates the case in which p is not inside a sandbox and therefore no privilege checks occur. As the graph shows, a single privilege check incurs approximately 5 microseconds of overhead. When sandboxes are nested, additional privilege checks incur approximately 1 microsecond each.

5 Related Work

Capabilities[1] are another alternative to sandboxes. A capability has two primary characteristics:

• A subject that holds a capability is granted access to the privilege it specifies.
  
  
• A subject that lacks a capability is denied access to the privilege it specifies.
  
  

Domain and type enforcement (DTE)[2, 3] is a useful tool for implementing mandatory access controls. This technique groups subjects into domains and objects into types. Rules are provided that specify which domains are granted access to which types. In addition, the system may be configured so that execution of certain programs causes transitions between domains. A major difference between DTE and our sandboxing mechanism is that DTE is geared toward implementing systemwide mandatory access controls. A trusted security administrator defines the domains and types, along with the rules governing their interactions. In contrast, sandboxes are lightweight entities that may be created, configured, and destroyed by untrusted users. Our implementation allows them to enforce either mandatory or discretionary access controls. We plan on extending their functionality by allowing transitions between sandboxes when certain programs are executed.

A variety of sandboxing techniques have been previously implemented. One approach is to build protection mechanisms into programming languages such as Java[7]. Since this option ties the sandbox to a particular language or runtime environment, it is not suitable as a general-purpose mechanism. However, it is still useful as a special-purpose technique, since security policies may be tailored to the language or runtime environment.

Alternately, the sandbox may be embedded within the sandboxed program. Proof-carrying code[8] is one example of this type of approach. Another option is to instrument an existing binary with additional machine instructions that verify compliance with a security policy as a program executes[9]. However, these alternatives are inconvenient because they require modification of binaries. Furthermore, they are not useful as general-purpose techniques since they do not apply to all types of programs (such as shell scripts, for instance).

A sandboxing system known as Janus[10], along with two similar mechanisms[11, 12], employs user-space monitoring processes for interception of system calls made by sandboxed programs. The monitoring processes use the /proc process tracing facility of Solaris for system call interception. This approach limits the scope of applicability of these techniques, since it may not be used with setuid programs. It also has substantial overhead because the monitoring agent is a separate process and interprocess context switches are therefore required for monitoring. Furthermore, the monitoring process must fork() each time the sandboxed process forks. The fact that the monitoring agent runs in user space may also create vulnerabilities.

To overcome the limitations of user-space mechanisms, sandboxes may be implemented as loadable kernel modules[14, 15]. Placing sandboxes inside the kernel may enhance their security by providing increased isolation from potentially malicious entities. Since kernel-based sandboxes may be implemented as passive entities, context switching overhead is not required for privilege checking. A disadvantage of this approach is that creating a new sandbox requires loading a kernel module. The module must be fully trusted, and a trusted user must perform the module loading operation.

A design known as ChakraVyuha (CV)[16] implements a kernel-based sandboxing mechanism. In this system, sandboxes for individual applications are defined using a domain-specific language. Sandbox definitions are stored in a secure location somewhere in the file system. When a given program is executed, its sandbox definition is passed to a kernel-resident enforcer. This entity enforces restrictions by matching system call parameters against the sandbox definition. Therefore, problems associated with implementing sandboxes as loadable kernel modules are avoided.

One difference between ChakraVyuha and our design is the level at which its external interfaces are specified. To confine a program with ChakraVyuha, it must first be installed using a specialized installer program. The installer generates a configuration file that specifies a default sandbox for the new program. If users wish to create customized sandboxes, they must do so using configuration files that follow a specific format. Our external interface is at a much lower level. We export a general-purpose system call API that application programs may use for their own purposes. This approach widens the scope of applicability of our design.

A second advantage of our model is the ability to dynamically reconfigure sandboxes at runtime. With ChakraVyuha, users may customize sandboxes, but the sandboxes are fixed once the sandboxed programs start executing. Other advantages of our model include nested sandboxes and our treatment of privilege sets as first class objects that may be manipulated using set-theoretic primitives.

Another solution, known as WindowBox[13], implements a sandboxing mechanism within the Windows NT kernel. The emphasis here is on providing an easy to use mechanism that is simple enough for unsophisticated users. The design consists of a set of desktops that are completely separated from each other and from the rest of the system. Users can give some desktops more privileges than others. As a user's level of trust increases, a program may be gradually moved to more privileged desktops. However, the desktops are relatively static entities. They are not designed to function as lightweight containers for individual programs.

Finally, a sandboxing mechanism somewhat similar to ours has been added to the ULTRIX operating system[17]. This mechanism, known as TRON, is similar to our design in some ways but more limited in scope, since it only deals with file-related privileges. Like our sandboxing mechanism, TRON allows creation of sandboxes by untrusted users. However, it does not provide a blocking mechanism for interactive privilege determination at runtime.

TRON does allow nesting of sandboxes, although this feature behaves differently from our design. When sandboxes are nested, our mechanism performs privilege checks at each level individually. However, TRON verifies at creation time that a nested sandbox contains a subset of its parent's privileges. It then checks privileges against only the innermost sandbox. Although TRON's approach reduces performance overhead, we chose our method for two reasons. First of all, our design allows changes in a sandbox configuration to affect all sandboxes nested below it. This behavior is necessary for interactive manipulation of sandboxes to function properly when sandboxes are nested. Secondly, our design allows a sandboxed process to create a nested sandbox without any awareness of how its own sandbox is configured. The child sandbox is not cluttered with restrictions imposed by its parent and therefore maintains a precise representation of the policies its creator wishes to enforce. Furthermore, restrictions imposed by the parent sandbox may be kept secret from its inhabitants.

The method that TRON employs for specifying access controls is less expressive than our file system component. When privileges are assigned to a directory, they automatically extend to all files it contains. It is not possible to grant privileges only for the directory without extending them to all of its files. However, a subtree option does exist that is equivalent to the union of self, children, and grandchild subtrees in our file system component. One feature that TRON omits is the ability to specifically deny access to files. It is therefore not powerful enough to support composition of privilege sets through union, intersection, and complement operations.

6 Conclusions

Notes

¹ Active entities are separate processes or threads that monitor the activities of sandboxed programs. Passive entities are variables or data structures maintained by the sandbox that are examined as part of the privilege checking steps that occur when a program attempts some action.

² Actually, a sandbox has two sets of attachment points for the various component types. The purpose of the second set of attachment points will be described later.

³ Here, we mean separate from the sandboxed program and any runtime environment in which it may be executing.

⁴ This assumes that file permission bits and other applicable security mechanisms also allow the operation.

⁵ Pools are collections of sandboxes. They will be described in more depth later.

⁶ semaphores, message queues, and shared memory segments

Availability

References

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