[v7,0/7] landlock: truncate support

Message

Günther Noack Sept. 30, 2022, 4:01 p.m. UTC

The goal of these patches is to work towards a more complete coverage
of file system operations that are restrictable with Landlock.

Motivation
----------

The known set of currently unsupported file system operations in
Landlock is described at [1]. Out of the operations listed there,
truncate is the only one that modifies file contents, so these patches
should make it possible to prevent the direct modification of file
contents with Landlock.

Apart from Landlock, file truncation can also be restricted using
seccomp-bpf, but it is more difficult to use (requires BPF, requires
keeping up-to-date syscall lists) and it is not configurable by file
hierarchy, as Landlock is. The simplicity and flexibility of the
Landlock approach makes it worthwhile adding.

Implementation overview
-----------------------

The patch introduces the truncation restriction feature as an
additional bit in the access_mask_t bitmap, in line with the existing
supported operations.

The truncation flag covers both the truncate(2) and ftruncate(2)
families of syscalls, as well as open(2) with the O_TRUNC flag.
This includes usages of creat() in the case where existing regular
files are overwritten.

Additionally, this patch set introduces a new Landlock security blob
associated with opened files, to track the available Landlock access
rights at the time of opening the file. This is in line with Unix's
general approach of checking the read and write permissions during
open(), and associating this previously checked authorization with the
opened file.

In order to treat truncate(2) and ftruncate(2) calls differently in an
LSM hook, we split apart the existing security_path_truncate hook into
security_path_truncate (for truncation by path) and
security_file_truncate (for truncation of previously opened files).

Relationship between "truncate" and "write" rights
--------------------------------------------------

While it's possible to use the "write file" and "truncate" rights
independent of each other, it simplifies the mental model for
userspace callers to always use them together.

Specifically, the following behaviours might be surprising for users
when using these independently:

 * The commonly creat() syscall requires the truncate right when
   overwriting existing files, as it is equivalent to open(2) with
   O_TRUNC|O_CREAT|O_WRONLY.
 * The "write file" right is not always required to truncate a file,
   even through the open(2) syscall (when using O_RDONLY|O_TRUNC).

Nevertheless, keeping the two flags separate is the correct approach
to guarantee backwards compatibility for existing Landlock users.

When the "truncate" right is checked for ftruncate(2)
-----------------------------------------------------

Notably, for the purpose of ftruncate(2), the Landlock truncation
access right is looked up when *opening* the file, not when calling
ftruncate(). The availability of the truncate right is associated with
the opened file and is later checked to authorize ftruncate(2)
operations.

This is similar to how the write mode gets remembered after a
open(..., O_WRONLY) to authorize later write() operations.

These opened file descriptors can also be passed between processes and
will continue to enforce their truncation properties when these
processes attempt an ftruncate().

Ongoing discussions
-------------------

Two main discussions that have still been ongoing recently on the v6
thread were:

* The question whether we need to touch fs/ksmbd and fs/cachefiles,
  which are both using vfs_truncate() to truncate files by path, even
  though they already have the same struct file open. The proposal was
  to introduce a "vfs_ftruncate()" that would work on opened files.

  I think we should decouple this from the truncate patch set, with
  the reasoning that:

  (a) it would be a bigger change to create a "vfs_ftruncate()" which
  would reach beyond the scope of this patch set.

  (b) it seems likely that both components do not need to run under
  Landlock at the moment and can be updated independently (just like
  it needs to happen for normal userspace software in order to run it
  under Landlock).

  (c) vfs_truncate() is not the perfectly narrowest API for truncating
  an opened file, but it's a legitimate way to do that and the
  operation *is* checked with a Landlock LSM hook, although it might
  potentially permit for a narrower sandboxing if that was done
  differently. That's speculative though.

  Overall, it's unclear whether doing this has any sandboxing benefits
  for ksmbd and cachefiles, whereas on the downside, it would expand
  the scope of the patch set quite a bit and would have to touch core
  parts of the kernel (fs/open.c).

* The question to what extent we should further refactor
  check_access_path_dual() and other functions in
  security/landlock/fs.c.

  I am personally fine with the state of the patch set as it is, but
  have proposed some possible refactorings on the v6 discussion.

  I'm fine with smaller changes, but I would like to avoid refactoring
  the core of the "refer" logic (for hardlink and rename operations)
  as part of this patch set. I think the chances of me getting this
  refactoring wrong on the first few attempts are high, and it seems
  it might not be worth holding off the truncate patch set because of
  it. At the same time, it seems like a reasonable patch set that
  could also stand on its own.

These patches are based on version 6.0-rc7.

Best regards,
Günther

[1] https://docs.kernel.org/userspace-api/landlock.html#filesystem-flags

Past discussions:
V1: https://lore.kernel.org/all/[e-mail]/
V2: https://lore.kernel.org/all/[e-mail]/
V3: https://lore.kernel.org/all/[e-mail]/
V4: https://lore.kernel.org/all/[e-mail]/
V5: https://lore.kernel.org/all/[e-mail]/
V6: https://lore.kernel.org/all/[e-mail]/

Changelog:

V7:
* security: Create file_truncate hook
  * Fix the build on kernels without CONFIG_SECURITY_PATH (fixed by
    Mickaël Salaün)
  * lsm_hooks.h: Document file_truncate hook
  * fs/open.c: undo accidental indentation changes
* landlock: Support file truncation
  * Use the init_layer_masks() result as input for
    check_access_path_dual()
  * Naming
    * Rename the landlock_file_security.allowed_access field
      (previously called "rights")
    * Rename get_path_access_rights() to get_path_access()
    * Rename get_file_access() to get_required_file_open_access() to
      avoid confusion with get_path_access()
    * Use "access_request" for access_mask_t variables, access_req for
      unsigned long
  * Documentation:
    * Fixed some comments according to review
    * Added comments to get_required_file_open_access() and
      init_layer_masks()
* selftests:
  * remove unused variables
  * rename fd0,...,fd3 to fd_layer0,...,fd_layer3.
  * test_ftruncate: define layers on top and inline the helper function
* New tests (both added as separate commits)
  * More exhaustive ftruncate test: Add open_and_ftruncate test that
    exercises ftruncate more thoroughly with fixture variants
  * FD-passing test: exercise restricted file descriptors getting
    passed between processes, also using the same fixture variants
* Documentation: integrate review comments by Mickaël Salaün
  * do not use contraptions (don't)
  * use double backquotes in all touched lines
  * add the read/write open() analogy to the truncation docs
  * in code example, check for abi<0 explicitly and fix indentation

V6:
* LSM hooks: create file_truncate hook in addition to path_truncate.
  Use it in the existing path_truncate call sites where appropriate.
* landlock: check LANDLOCK_ACCESS_FS_TRUNCATE right during open(), and
  associate that right with the opened struct file in a security blob.
  Introduce get_path_access_rights() helper function.
* selftests: test ftruncate in a separate test, to exercise that
  the rights are associated with the file descriptor.
* Documentation: Rework documentation to reflect new ftruncate() semantics.
* Applied small fixes by Mickaël Salaün which he added on top of V5, in
  https://git.kernel.org/pub/scm/linux/kernel/git/mic/linux.git/log/?h=next
  (I hope I found them all.)

V5:
* Documentation
  * Fix wording in userspace-api headers and in landlock.rst.
  * Move the truncation limitation section one to the bottom.
  * Move all .rst changes into the documentation commit.
* selftests
  * Remove _metadata argument from helpers where it became unnecessary.
  * Open writable file descriptors at the top of both tests, before Landlock
    is enabled, to exercise ftruncate() independently from open().
  * Simplify test_ftruncate and decouple it from exercising open().
  * test_creat(): Return errno on close() failure (it does not conflict).
  * Fix /* comment style */
  * Reorder blocks of EXPECT_EQ checks to be consistent within a test.
  * Add missing |O_TRUNC to a check in one test.
  * Put the truncate_unhandled test before the other.

V4:
 * Documentation
   * Clarify wording and syntax as discussed in review.
   * Use a less confusing error message in the example.
 * selftests:
   * Stop using ASSERT_EQ in test helpers, return EBADFD instead.
     (This is an intentionally uncommon error code, so that the source
     of the error is clear and the test can distinguish test setup
     failures from failures in the actual system call under test.)
 * samples/Documentation:
   * Use additional clarifying comments in the kernel backwards
     compatibility logic.

V3:
 * selftests:
   * Explicitly test ftruncate with readonly file descriptors
     (returns EINVAL).
   * Extract test_ftruncate, test_truncate, test_creat helpers,
     which simplified the previously mixed usage of EXPECT/ASSERT.
   * Test creat() behaviour as part of the big truncation test.
   * Stop testing the truncate64(2) and ftruncate64(2) syscalls.
     This simplifies the tests a bit. The kernel implementations are the
     same as for truncate(2) and ftruncate(2), so there is little benefit
     from testing them exhaustively. (We aren't testing all open(2)
     variants either.)
 * samples/landlock/sandboxer.c:
   * Use switch() to implement best effort mode.
 * Documentation:
   * Give more background on surprising truncation behaviour.
   * Use switch() in the example too, to stay in-line with the sample tool.
   * Small fixes in header file to address previous comments.
* misc:
  * Fix some typos and const usages.

V2:
 * Documentation: Mention the truncation flag where needed.
 * Documentation: Point out connection between truncation and file writing.
 * samples: Add file truncation to the landlock/sandboxer.c sample tool.
 * selftests: Exercise open(2) with O_TRUNC and creat(2) exhaustively.
 * selftests: Exercise truncation syscalls when the truncate right
   is not handled by Landlock.

Günther Noack (7):
  security: Create file_truncate hook from path_truncate hook
  landlock: Support file truncation
  selftests/landlock: Selftests for file truncation support
  selftests/landlock: Test open() and ftruncate() in multiple scenarios
  selftests/landlock: Test FD passing from a Landlock-restricted to an
    unrestricted process
  samples/landlock: Extend sample tool to support
    LANDLOCK_ACCESS_FS_TRUNCATE
  landlock: Document Landlock's file truncation support

 Documentation/userspace-api/landlock.rst     |  66 ++-
 fs/namei.c                                   |   2 +-
 fs/open.c                                    |   2 +-
 include/linux/lsm_hook_defs.h                |   1 +
 include/linux/lsm_hooks.h                    |  10 +-
 include/linux/security.h                     |   6 +
 include/uapi/linux/landlock.h                |  21 +-
 samples/landlock/sandboxer.c                 |  23 +-
 security/apparmor/lsm.c                      |   6 +
 security/landlock/fs.c                       | 117 ++++-
 security/landlock/fs.h                       |  24 +
 security/landlock/limits.h                   |   2 +-
 security/landlock/setup.c                    |   1 +
 security/landlock/syscalls.c                 |   2 +-
 security/security.c                          |   5 +
 security/tomoyo/tomoyo.c                     |  13 +
 tools/testing/selftests/landlock/base_test.c |  38 +-
 tools/testing/selftests/landlock/common.h    |  67 +++
 tools/testing/selftests/landlock/fs_test.c   | 452 ++++++++++++++++++-
 19 files changed, 789 insertions(+), 69 deletions(-)


base-commit: f76349cf41451c5c42a99f18a9163377e4b364ff