systemd

Using /tmp/ And /var/tmp/ Safely

/tmp/ and /var/tmp/ are two world-writable directories Linux systems provide for temporary files. The former is typically on tmpfs and thus backed by RAM/swap, and flushed out on each reboot. The latter is typically a proper, persistent file system, and thus backed by physical storage. This means:

  1. /tmp/ should be used for smaller, size-bounded files only; /var/tmp/ should be used for everything else.

  2. Data that shall survive a boot cycle shouldn’t be placed in /tmp/.

If the $TMPDIR environment variable is set, use that path, and neither use /tmp/ nor /var/tmp/ directly.

See file-hierarchy(7) for details about these two (and most other) directories of a Linux system.

Common Namespace

Note that /tmp/ and /var/tmp/ each define a common namespace shared by all local software. This means guessable file or directory names below either directory directly translate into a 🚨 Denial-of-Service (DoS) 🚨 vulnerability or worse: if some software creates a file or directory /tmp/foo then any other software that wants to create the same file or directory /tmp/foo either will fail (as the file already exists) or might be tricked into using untrusted files. Hence: do not use guessable names in /tmp/ or /var/tmp/ — if you do you open yourself up to a local DoS exploit or worse. (You can get away with using guessable names, if you pre-create subdirectories below /tmp/ for them, like X11 does with /tmp/.X11-unix/ through tmpfiles.d/ drop-ins. However this is not recommended, as it is fully safe only if these directories are pre-created during early boot, and thus problematic if package installation during runtime is permitted.)

To protect yourself against these kinds of attacks Linux provides a couple of APIs that help you avoiding guessable names. Specifically:

  1. Use mkstemp() (POSIX), mkostemp() (glibc), mkdtemp() (POSIX), tmpfile() (C89)

  2. Use open() with O_TMPFILE (Linux)

  3. memfd_create() (Linux; this doesn’t bother with /tmp/ or /var/tmp/ at all, but uses the same RAM/swap backing as tmpfs uses, hence is very similar to /tmp/ semantics.)

For system services systemd provides the PrivateTmp= boolean setting. If turned on for a service (👍 which is highly recommended), /tmp/ and /var/tmp/ are replaced by private sub-directories, implemented through Linux file system namespacing and bind mounts. This means from the service’s point of view /tmp/ and /var/tmp/ look and behave like they normally do, but in reality they are private sub-directories of the host’s real /tmp/ and /var/tmp/, and thus not system-wide locations anymore, but service-specific ones. This reduces the surface for local DoS attacks substantially. While it is recommended to turn this option on, it’s highly recommended for applications not to rely on this solely to avoid DoS vulnerabilities, because this option is not available in environments where file system namespaces are prohibited, for example in certain container environments. This option is hence an extra line of defense, but should not be used as an excuse to rely on guessable names in /tmp/ and /var/tmp/. When this option is used, the per-service temporary directories are removed whenever the service shuts down, hence the lifecycle of temporary files stored in it is substantially different from the case where this option is not used. Also note that some applications use /tmp/ and /var/tmp/ for sharing files and directories. If this option is turned on this is not possible anymore as after all each service gets its own instances of both directories.

Automatic Clean-Up

By default, systemd-tmpfiles will apply a concept of ⚠️ “ageing” to all files and directories stored in /tmp/ and /var/tmp/. This means that files that have neither been changed nor read within a specific time frame are automatically removed in regular intervals. (This concept is not new to systemd-tmpfiles btw, it’s inherited from previous subsystems such as tmpwatch.) By default files in /tmp/ are cleaned up after 10 days, and those in /var/tmp after 30 days.

This automatic clean-up is important to ensure disk usage of these temporary directories doesn’t grow without bounds, even when programs abort unexpectedly or otherwise don’t clean up the temporary files/directories they create. On the other hand it creates problems for long-running software that does not expect temporary files it operates on to be suddenly removed. There are a couple of strategies to avoid these issues:

  1. Make sure to always keep a file descriptor to the temporary files you operate on open, and only access the files through them. This way it doesn’t matter whether the files have been unlinked from the file system: as long as you have the file descriptor open you can still access the file for both reading and writing. When operating this way it is recommended to delete the files right after creating them to ensure that on unexpected program termination the files or directories are implicitly released by the kernel.

  2. 🥇 Use memfd_create() or O_TMPFILE. This is an extension of the suggestion above: files created this way are never linked under a filename in the file system. This means they are not subject to ageing (as they come unlinked out of the box), and there’s no time window where a directory entry for the file exists in the file system, and thus behaviour is fully robust towards unexpected program termination as there are never files on disk that need to be explicitly deleted.

  3. 🥇 Operate below a sub-directory of /tmp/ and /var/tmp/ you created, and take a BSD file lock (flock(dir_fd, LOCK_SH)) on that sub-directory. This is particularly interesting when operating on more than a single file, or on file nodes that are not plain regular files, for example when extracting a tarball to a temporary directory. The ageing algorithm will skip all directories (and everything below them) that are locked through a BSD file lock. As BSD file locks are automatically released when the file descriptor they are taken on is closed, and all file descriptors opened by a process are implicitly closed when it exits, this is a robust mechanism that ensures all temporary files are subject to ageing when the program that owns them dies, but not while it is still running. Use this when decompressing tarballs that contain files with old modification/access times, as extracted files are otherwise immediately candidates for deletion by the ageing algorithm. The flock tool of the util-linux packages makes this concept available to shell scripts. Note that systemd-tmpfiles only checks for BSD file locks on directories, locks on other types of file nodes (including regular files) are not considered.

  4. Keep the access time of all temporary files created current. In regular intervals, use utimensat() or a related call to update the access time (“atime”) of all files that shall be kept around. Since the ageing algorithm looks at the access time of files when deciding whether to delete them, it’s sufficient to update their access times in sufficiently frequent intervals to ensure the files are not deleted. Since most applications (and tools such as ls) primarily care for the modification time (rather than the access time) using the access time for this purpose should be acceptable.

  5. Set the “sticky” bit on regular files. The ageing logic skips deletion of all regular files that have the sticky bit (chmod +t) set. This is honoured for regular files only however, and has no effect on directories as the sticky bit has a different meaning for them.

  6. Don’t use /tmp/ or /var/tmp/, but use your own sub-directory under /run/ or $XDG_RUNTIME_DIRECTORY (the former if privileged, the latter if unprivileged), or /var/lib/ and ~/.config/ (similar, but with persistency and suitable for larger data). The two temporary directories /tmp/ and /var/tmp/ come with the implicit clean-up semantics described above. When this is not desired, it’s possible to create private per-package runtime or state directories, and place all temporary files there. However, do note that this means opting out of any kind of automatic clean-up, and it is hence particularly essential that the program cleans up generated files in these directories when they are no longer needed, in particular when the program dies unexpectedly. Note: this strategy is only really suitable for packages that operate in a “system wide singleton” fashion with “long” persistence of its data or state, i.e. as opposed to programs that run in multiple parallel or short-living instances. This is because a private directory under /run (and the other mentioned directories) is itself system and package specific singleton with greater longevity.

  7. Exclude your temporary files from clean-ups via a tmpfiles.d/ drop-in (which includes drop-ins in the runtime-only directory /run/tmpfiles.d/). The x/X line types may be used to exclude files matching the specified globbing patterns from the ageing logic. If this is used, automatic clean-up is not done for matching files and directory, and much like with the previous option it’s hence essential that the program generating these temporary files carefully removes the temporary files it creates again, and in particular so if it dies unexpectedly.

🥇 The semantics of options 2 (in case you only deal with temporary files, not directories) and 3 (in case you deal with both) in the list above are in most cases the most preferable. It is thus recommended to stick to these two options.

While the ageing logic is very useful as a safety concept to ensure unused files and directories are eventually removed a well written program avoids even creating files that need such a clean-up. In particular:

  1. Use memfd_create() or O_TMPFILE when creating temporary files.

  2. unlink() temporary files right after creating them. This is very similar to O_TMPFILE behaviour: consider deleting temporary files right after creating them, while keeping open a file descriptor to them. Unlike O_TMPFILE this method also works on older Linux systems and other OSes that do not implement O_TMPFILE.

Disk Quota

Generally, files allocated from /tmp/ and /var/tmp/ are allocated from a pool shared by all local users. Moreover the space available in /tmp/ is generally more restricted than /var/tmp/. This means, that in particular in /tmp/ space should be considered scarce, and programs need to be prepared that no space is available. Essential programs might require a fallback logic using a different location for storing temporary files hence. Non-essential programs at least need to be prepared for ENOSPC errors and generate useful, actionable error messages.

Some setups employ per-user quota on /var/tmp/ and possibly /tmp/, to make ENOSPC situations less likely, and harder to trigger from unprivileged users. However, in the general case no such per-user quota is implemented though, in particular not when tmpfs is used as backing file system, because — even today — tmpfs still provides no native quota support in the kernel.

Early Boot Considerations

Both /tmp/ and /var/tmp/ are not necessarily available during early boot, or — if they are available early — are not writable. This means software that is intended to run during early boot (i.e. before basic.target — or more specifically local-fs.target — is up) should not attempt to make use of either. Interfaces such as memfd_create() or files below a package-specific directory in /run/ are much better options in this case. (Note that some packages instead use /dev/shm/ for temporary files during early boot; this is not advisable however, as it offers no benefits over a private directory in /run/ as both are backed by the same concept: tmpfs. The directory /dev/shm/ exists to back POSIX shared memory (see shm_open() and related calls), and not as a place for temporary files. /dev/shm is problematic as it is world-writable and there’s no automatic clean-up logic in place.)