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@xxxxxxxxxxx
Previous versions:
v2: https://lore.kernel.org/r/20220329125117.1393824-1-mic@xxxxxxxxxxx
v1: https://lore.kernel.org/r/20220221212522.320243-1-mic@xxxxxxxxxxx
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
