OpenBSD's unveil()

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One of the key aspects of hardening the user-space side of an operating system is to provide mechanisms for restricting which parts of the filesystem hierarchy a given process can access. Linux has a number of mechanisms of varying capability and complexity for this purpose, but other kernels have taken a different approach. Over the last few months, OpenBSD has inaugurated a new system call named unveil() for this type of hardening that differs significantly from the mechanisms found in Linux.

The value of restricting access to the filesystem, from a security point of view, is fairly obvious. A compromised process cannot exfiltrate data that it cannot read, and it cannot corrupt files that it cannot write. Preventing unwanted access is, of course, the purpose of the permissions bits attached to every file, but permissions fall short in an important way: just because a particular user has access to a given file does not necessarily imply that every program run by that user should also have access to that file. There is no reason why your PDF viewer should be able to read your SSH keys, for example. Relying on just the permission bits makes it easy for a compromised process to access files that have nothing to do with that process's actual job.

In a Linux system, there are many ways of trying to restrict that access; that is one of the purposes behind the Linux security module (LSM) architecture, for example. The SELinux LSM uses a complex matrix of labels and roles to make access-control decisions. The AppArmor LSM, instead, uses a relatively simple table of permissible pathnames associated with each application; that approach was highly controversial when AppArmor was first merged, and is still looked down upon by some security developers. Mount namespaces can be used to create a special view of the filesystem hierarchy for a set of processes, rendering much of that hierarchy invisible and, thus, inaccessible. The seccomp mechanism can be used to make decisions on attempts by a process to access files, but that approach is complex and error-prone. Yet another approach can be seen in the Qubes OS distribution, which runs applications in virtual machines to strictly control what they can access.

Compared to many of the options found in Linux, unveil() is an exercise in simplicity. This system call, introduced in July, has this prototype:

int unveil(const char *path, const char *permissions);

A process that has never called unveil() has full access to the filesystem hierarchy, modulo the usual file permissions and any restrictions that may have been applied by calling pledge() . Calling unveil() for the first time will "drop a veil" across the entire filesystem, rendering the whole thing invisible to the process, with one exception: the file or directory hierarchy starting at path will be accessible with the given permissions . The permissions string can contain any of " r " for read access, " w " for write, " x " for execute, and " c " for the ability to create or remove the path .

Subsequent calls to unveil() will make other parts of the filesystem hierarchy accessible; the unveil() system call itself still has access to the entire hierarchy, so there is no problem with unveiling distinct subtrees that are, until the call is made, invisible to the process. If one unveil() call applies to a subtree of a hierarchy unveiled by another call, the permissions associated with the more specific call apply.

Calling unveil() with both arguments as null will block any further calls, setting the current view of the filesystem in stone. Calls to unveil() can also be blocked using pledge() . Either way, once the view of the filesystem has been set up appropriately, it is possible to lock it so that the process cannot expand its access in the future should it be taken over and turn hostile.

unveil() thus looks a bit like AppArmor, in that it is a path-based mechanism for restricting access to files. In either case, one must first study the program in question to gain a solid understanding of which files it needs to access before closing things down, or the program is likely to break. One significant difference (beyond the other sorts of behavior that AppArmor can control) is that AppArmor's permissions are stored in an external policy file, while unveil() calls are made by the application itself. That approach keeps the access rules tightly tied to the application and easy for the developers to modify, but it also makes it harder for system administrators to change them without having to rebuild the application from source.

One can certainly aim a number of criticisms at unveil() — all of the complaints that have been leveled at path-based access control and more. But the simplicity of unveil() brings a certain kind of utility, as can be seen in the large number of OpenBSD applications that are being modified to use it. OpenBSD is gaining a base level of protection against unintended program behavior; while it is arguably possible to protect a Linux system to a much greater extent, the complexity of the mechanisms involved keeps that from happening in a lot of real-world deployments. There is a certain kind of virtue to simplicity in security mechanisms.

