Hello! These patches add simple ioctl(2) support to Landlock. Objective ~~~~~~~~~ Make ioctl(2) requests restrictable with Landlock, in a way that is useful for real-world applications. Proposed approach ~~~~~~~~~~~~~~~~~ Introduce the LANDLOCK_ACCESS_FS_IOCTL right, which restricts the use of ioctl(2) on file descriptors. We attach IOCTL access rights to opened file descriptors, as we already do for LANDLOCK_ACCESS_FS_TRUNCATE. If LANDLOCK_ACCESS_FS_IOCTL is handled (restricted in the ruleset), the LANDLOCK_ACCESS_FS_IOCTL access right governs the use of all IOCTL commands. We make an exception for the common and known-harmless IOCTL commands FIOCLEX, FIONCLEX, FIONBIO and FIONREAD. These IOCTL commands are always permitted. Their functionality is already available through fcntl(2). If additionally(!), the access rights LANDLOCK_ACCESS_FS_READ_FILE, LANDLOCK_ACCESS_FS_WRITE_FILE or LANDLOCK_ACCESS_FS_READ_DIR are handled, these access rights also unlock some IOCTL commands which are considered safe for use with files opened in these ways. As soon as these access rights are handled, the affected IOCTL commands can not be permitted through LANDLOCK_ACCESS_FS_IOCTL any more, but only be permitted through the respective more specific access rights. A full list of these access rights is listed below in this cover letter and in the documentation. I believe that this approach works for the majority of use cases, and offers a good trade-off between Landlock API and implementation complexity and flexibility when the feature is used. Current limitations ~~~~~~~~~~~~~~~~~~~ With this patch set, ioctl(2) requests can *not* be filtered based on file type, device number (dev_t) or on the ioctl(2) request number. On the initial RFC patch set [1], we have reached consensus to start with this simpler coarse-grained approach, and build additional IOCTL restriction capabilities on top in subsequent steps. [1] https://lore.kernel.org/linux-security-module/d4f1395c-d2d4-1860-3a02-2a0c023dd761@xxxxxxxxxxx/ Notable implications of this approach ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Existing inherited file descriptors stay unaffected when a program enables Landlock. This means in particular that in common scenarios, the terminal's IOCTLs (ioctl_tty(2)) continue to work. * ioctl(2) continues to be available for file descriptors acquired through means other than open(2). Example: Network sockets, memfd_create(2), file descriptors that are already open before the Landlock ruleset is enabled. Examples ~~~~~~~~ Starting a sandboxed shell from $HOME with samples/landlock/sandboxer: LL_FS_RO=/ LL_FS_RW=. ./sandboxer /bin/bash The LANDLOCK_ACCESS_FS_IOCTL right is part of the "read-write" rights here, so we expect that newly opened files outside of $HOME don't work with most IOCTL commands. * "stty" works: It probes terminal properties * "stty </dev/tty" fails: /dev/tty can be reopened, but the IOCTL is denied. * "eject" fails: ioctls to use CD-ROM drive are denied. * "ls /dev" works: It uses ioctl to get the terminal size for columnar layout * The text editors "vim" and "mg" work. (GNU Emacs fails because it attempts to reopen /dev/tty.) IOCTL groups ~~~~~~~~~~~~ To decide which IOCTL commands should be blanket-permitted we went through the list of IOCTL commands mentioned in fs/ioctl.c and looked at them individually to understand what they are about. The following list is for reference. We should always allow the following IOCTL commands, which are also available through fcntl(2) with the F_SETFD and F_SETFL commands: * FIOCLEX, FIONCLEX - these work on the file descriptor and manipulate the close-on-exec flag * FIONBIO, FIOASYNC - these work on the struct file and enable nonblocking-IO and async flags The following command is guarded and enabled by either of LANDLOCK_ACCESS_FS_WRITE_FILE, LANDLOCK_ACCESS_FS_READ_FILE or LANDLOCK_ACCESS_FS_READ_DIR (G2), once one of them is handled (otherwise by LANDLOCK_ACCESS_FS_IOCTL): * FIOQSIZE - get the size of the opened file The following commands are guarded and enabled by either of LANDLOCK_ACCESS_FS_WRITE_FILE or LANDLOCK_ACCESS_FS_READ_FILE (G2), once one of them is handled (otherwise by LANDLOCK_ACCESS_FS_IOCTL): These are commands that read file system internals: * FS_IOC_FIEMAP - get information about file extent mapping (c.f. https://www.kernel.org/doc/Documentation/filesystems/fiemap.txt) * FIBMAP - get a file's file system block number * FIGETBSZ - get file system blocksize The following commands are guarded and enabled by LANDLOCK_ACCESS_FS_READ_FILE (G3), if it is handled (otherwise by LANDLOCK_ACCESS_FS_IOCTL): * FIONREAD - get the number of bytes available for reading (the implementation is defined per file type) * FIDEDUPRANGE - manipulating shared physical storage between files. The following commands are guarded and enabled by LANDLOCK_ACCESS_FS_WRITE_FILE (G4), if it is handled (otherwise by LANDLOCK_ACCESS_FS_IOCTL): * FICLONE, FICLONERANGE - making files share physical storage between multiple files. These only work on some file systems, by design. * FS_IOC_RESVSP, FS_IOC_RESVSP64, FS_IOC_UNRESVSP, FS_IOC_UNRESVSP64, FS_IOC_ZERO_RANGE: Backwards compatibility with legacy XFS preallocation syscalls which predate fallocate(2). The following commands are also mentioned in fs/ioctl.c, but are not handled specially and are managed by LANDLOCK_ACCESS_FS_IOCTL together with all other remaining IOCTL commands: * FIFREEZE, FITHAW - work on superblock(!) to freeze/thaw the file system. Requires CAP_SYS_ADMIN. * Accessing file attributes: * FS_IOC_GETFLAGS, FS_IOC_SETFLAGS - manipulate inode flags (ioctl_iflags(2)) * FS_IOC_FSGETXATTR, FS_IOC_FSSETXATTR - more attributes Open questions ~~~~~~~~~~~~~~ This is unlikely to be the last iteration, but we are getting closer. Some notable open questions are: * Code style * Should we move the IOCTL access right expansion logic into the outer layers in syscall.c? Where it currently lives in ruleset.h, this logic feels too FS-specific, and it introduces the additional complication that we now have to track which access_mask_t-s are already expanded and which are not. It might be simpler to do the expansion earlier. * Rename IOCTL_CMD_G1, ..., IOCTL_CMD_G4 and give them better names. * When LANDLOCK_ACCESS_FS_IOCTL is granted on a file hierarchy, should this grant the permission to use *any* IOCTL? (Right now, it is any IOCTL except for the ones covered by the IOCTL groups, and it's a bit weird that the scope of LANDLOCK_ACCESS_FS_IOCTL becomes smaller when other access rights are also handled. * Backwards compatibility for user-space libraries. This is not documented yet, because it is currently not necessary yet. But as soon as we have a hypothetical Landlock ABI v6 with a new IOCTL-enabled "GFX" access right, the "best effort" downgrade from v6 to v5 becomes more involved: If the caller handles GFX+IOCTL and permits GFX on a file, the correct downgrade to make this work on a Landlock v5 kernel is to handle IOCTL only, and permit IOCTL(!). Related Work ~~~~~~~~~~~~ OpenBSD's pledge(2) [2] restricts ioctl(2) independent of the file descriptor which is used. The implementers maintain multiple allow-lists of predefined ioctl(2) operations required for different application domains such as "audio", "bpf", "tty" and "inet". OpenBSD does not guarantee ABI backwards compatibility to the same extent as Linux does, so it's easier for them to update these lists in later versions. It might not be a feasible approach for Linux though. [2] https://man.openbsd.org/OpenBSD-7.3/pledge.2 Changes ~~~~~~~ V4: * use "synthetic" IOCTL access rights, as previously discussed * testing changes * use a large fixture-based test, for more exhaustive coverage, and replace some of the earlier tests with it * rebased on mic-next V3: * always permit the IOCTL commands FIOCLEX, FIONCLEX, FIONBIO, FIOASYNC and FIONREAD, independent of LANDLOCK_ACCESS_FS_IOCTL * increment ABI version in the same commit where the feature is introduced * testing changes * use FIOQSIZE instead of TTY IOCTL commands (FIOQSIZE works with regular files, directories and memfds) * run the memfd test with both Landlock enabled and disabled * add a test for the always-permitted IOCTL commands V2: * rebased on mic-next * added documentation * exercise ioctl(2) in the memfd test * test: Use layout0 for the test --- V1: https://lore.kernel.org/linux-security-module/20230502171755.9788-1-gnoack3000@xxxxxxxxx/ V2: https://lore.kernel.org/linux-security-module/20230623144329.136541-1-gnoack@xxxxxxxxxx/ V3: https://lore.kernel.org/linux-security-module/20230814172816.3907299-1-gnoack@xxxxxxxxxx/ Günther Noack (7): landlock: Optimize the number of calls to get_access_mask slightly landlock: Add IOCTL access right selftests/landlock: Test IOCTL support selftests/landlock: Test IOCTL with memfds selftests/landlock: Test ioctl(2) and ftruncate(2) with open(O_PATH) samples/landlock: Add support for LANDLOCK_ACCESS_FS_IOCTL landlock: Document IOCTL support Documentation/userspace-api/landlock.rst | 74 ++- include/uapi/linux/landlock.h | 51 +- samples/landlock/sandboxer.c | 10 +- security/landlock/fs.c | 74 ++- security/landlock/limits.h | 10 +- security/landlock/ruleset.c | 5 +- security/landlock/ruleset.h | 53 +- security/landlock/syscalls.c | 6 +- tools/testing/selftests/landlock/base_test.c | 2 +- tools/testing/selftests/landlock/fs_test.c | 498 ++++++++++++++++++- 10 files changed, 735 insertions(+), 48 deletions(-) base-commit: f12f8f84509a084399444c4422661345a15cc713 -- 2.42.0.869.gea05f2083d-goog
