| Message ID | 20220506161102.525323-1-mic@digikod.net (mailing list archive) |
|---|---|
| Headers | show |
| Series | Landlock: file linking and renaming support | expand |
The four related patch series are available here: https://git.kernel.org/pub/scm/linux/kernel/git/mic/linux.git/log/?h=landlock-wip On 06/05/2022 18:10, Mickaël Salaün wrote: > Hi, > > This third patch series is mostly a rebase with some whitespace changes > because of clang-format. There is also some new "unlikely()" calls and > minor code cleanup. > > Test coverage for security/landlock was 94.4% of 504 lines (with the > previous patch series), and it is now 95.4% of 604 lines according to > gcc/gcov-11. > > Problem > ======= > > One of the most annoying limitations of Landlock is that sandboxed > processes can only link and rename files to the same directory (i.e. > file reparenting is always denied). Indeed, because of the unprivileged > nature of Landlock, file hierarchy are identified thanks to ephemeral > inode tagging, which may cause arbitrary renaming and linking to change > the security policy in an unexpected way. > > Solution > ======== > > This patch series brings a new access right, LANDLOCK_ACCESS_FS_REFER, > which enables to allow safe file linking and renaming. In a nutshell, > Landlock checks that the inherited access rights of a moved or renamed > file cannot increase but only reduce. Eleven new test suits cover file > renaming and linking, which improves test coverage. > > The documentation and the tutorial is extended with this new access > right, along with more explanations about backward and forward > compatibility, good practices, and a bit about the current access > rights rational. > > While developing this new feature, I also found an issue with the > current implementation of Landlock. In some (rare) cases, sandboxed > processes may be more restricted than intended. Indeed, because of the > current way to check file hierarchy access rights, composition of rules > may be incomplete when requesting multiple accesses at the same time. > This is fixed with a dedicated patch involving some refactoring. A new > test suite checks relevant new edge cases. > > As a side effect, and to limit the increased use of the stack, I reduced > the number of Landlock nested domains from 64 to 16. I think this > should be more than enough for legitimate use cases, but feel free to > challenge this decision with real and legitimate use cases. > > Additionally, a new dedicated syzkaller test has been developed to cover > new paths. > > This patch series is based on and was developed with some complementary > new tests sent in a standalone patch series: > https://lore.kernel.org/r/20220506160820.524344-1-mic@digikod.net > > Previous versions: > v2: https://lore.kernel.org/r/20220329125117.1393824-1-mic@digikod.net > v1: https://lore.kernel.org/r/20220221212522.320243-1-mic@digikod.net > > Regards, > > Mickaël Salaün (12): > landlock: Define access_mask_t to enforce a consistent access mask > size > landlock: Reduce the maximum number of layers to 16 > landlock: Create find_rule() from unmask_layers() > landlock: Fix same-layer rule unions > landlock: Move filesystem helpers and add a new one > LSM: Remove double path_rename hook calls for RENAME_EXCHANGE > landlock: Add support for file reparenting with > LANDLOCK_ACCESS_FS_REFER > selftests/landlock: Add 11 new test suites dedicated to file > reparenting > samples/landlock: Add support for file reparenting > landlock: Document LANDLOCK_ACCESS_FS_REFER and ABI versioning > landlock: Document good practices about filesystem policies > landlock: Add design choices documentation for filesystem access > rights > > Documentation/security/landlock.rst | 17 +- > Documentation/userspace-api/landlock.rst | 151 ++- > include/linux/lsm_hook_defs.h | 2 +- > include/linux/lsm_hooks.h | 1 + > include/uapi/linux/landlock.h | 27 +- > samples/landlock/sandboxer.c | 40 +- > security/apparmor/lsm.c | 30 +- > security/landlock/fs.c | 771 ++++++++++--- > security/landlock/fs.h | 2 +- > security/landlock/limits.h | 6 +- > security/landlock/ruleset.c | 6 +- > security/landlock/ruleset.h | 22 +- > security/landlock/syscalls.c | 2 +- > security/security.c | 9 +- > security/tomoyo/tomoyo.c | 11 +- > tools/testing/selftests/landlock/base_test.c | 2 +- > tools/testing/selftests/landlock/fs_test.c | 1039 ++++++++++++++++-- > 17 files changed, 1853 insertions(+), 285 deletions(-) > > > base-commit: 4b0cdb0cf6eefa7521322007931ccfb7edc96c53
Hi, This third patch series is mostly a rebase with some whitespace changes because of clang-format. There is also some new "unlikely()" calls and minor code cleanup. Test coverage for security/landlock was 94.4% of 504 lines (with the previous patch series), and it is now 95.4% of 604 lines according to gcc/gcov-11. Problem ======= One of the most annoying limitations of Landlock is that sandboxed processes can only link and rename files to the same directory (i.e. file reparenting is always denied). Indeed, because of the unprivileged nature of Landlock, file hierarchy are identified thanks to ephemeral inode tagging, which may cause arbitrary renaming and linking to change the security policy in an unexpected way. Solution ======== This patch series brings a new access right, LANDLOCK_ACCESS_FS_REFER, which enables to allow safe file linking and renaming. In a nutshell, Landlock checks that the inherited access rights of a moved or renamed file cannot increase but only reduce. Eleven new test suits cover file renaming and linking, which improves test coverage. The documentation and the tutorial is extended with this new access right, along with more explanations about backward and forward compatibility, good practices, and a bit about the current access rights rational. While developing this new feature, I also found an issue with the current implementation of Landlock. In some (rare) cases, sandboxed processes may be more restricted than intended. Indeed, because of the current way to check file hierarchy access rights, composition of rules may be incomplete when requesting multiple accesses at the same time. This is fixed with a dedicated patch involving some refactoring. A new test suite checks relevant new edge cases. As a side effect, and to limit the increased use of the stack, I reduced the number of Landlock nested domains from 64 to 16. I think this should be more than enough for legitimate use cases, but feel free to challenge this decision with real and legitimate use cases. Additionally, a new dedicated syzkaller test has been developed to cover new paths. This patch series is based on and was developed with some complementary new tests sent in a standalone patch series: https://lore.kernel.org/r/20220506160820.524344-1-mic@digikod.net Previous versions: v2: https://lore.kernel.org/r/20220329125117.1393824-1-mic@digikod.net v1: https://lore.kernel.org/r/20220221212522.320243-1-mic@digikod.net Regards, Mickaël Salaün (12): landlock: Define access_mask_t to enforce a consistent access mask size landlock: Reduce the maximum number of layers to 16 landlock: Create find_rule() from unmask_layers() landlock: Fix same-layer rule unions landlock: Move filesystem helpers and add a new one LSM: Remove double path_rename hook calls for RENAME_EXCHANGE landlock: Add support for file reparenting with LANDLOCK_ACCESS_FS_REFER selftests/landlock: Add 11 new test suites dedicated to file reparenting samples/landlock: Add support for file reparenting landlock: Document LANDLOCK_ACCESS_FS_REFER and ABI versioning landlock: Document good practices about filesystem policies landlock: Add design choices documentation for filesystem access rights Documentation/security/landlock.rst | 17 +- Documentation/userspace-api/landlock.rst | 151 ++- include/linux/lsm_hook_defs.h | 2 +- include/linux/lsm_hooks.h | 1 + include/uapi/linux/landlock.h | 27 +- samples/landlock/sandboxer.c | 40 +- security/apparmor/lsm.c | 30 +- security/landlock/fs.c | 771 ++++++++++--- security/landlock/fs.h | 2 +- security/landlock/limits.h | 6 +- security/landlock/ruleset.c | 6 +- security/landlock/ruleset.h | 22 +- security/landlock/syscalls.c | 2 +- security/security.c | 9 +- security/tomoyo/tomoyo.c | 11 +- tools/testing/selftests/landlock/base_test.c | 2 +- tools/testing/selftests/landlock/fs_test.c | 1039 ++++++++++++++++-- 17 files changed, 1853 insertions(+), 285 deletions(-) base-commit: 4b0cdb0cf6eefa7521322007931ccfb7edc96c53