[PATCH v4 0/2] landlock: Fix TSYNC deadlock and clean up error path

[Yihan Ding <dingyihan@uniontech.com>]

Hello,

This patch series fixes a deadlock in the Landlock TSYNC multithreading 
support, originally reported by syzbot, and cleans up the associated 
interrupt recovery path.

The deadlock occurs when multiple threads concurrently call 
landlock_restrict_self() with sibling thread restriction enabled, 
causing them to mutually queue task_works on each other and block 
indefinitely.

* Patch 1 fixes the root cause by serializing the TSYNC operations 
  within the same process using the exec_update_lock.
* Patch 2 cleans up the interrupt recovery path by replacing an 
  unnecessary wait_for_completion() with a straightforward loop break, 
  avoiding Use-After-Free while unblocking remaining task_works.

Changes in v4:
- Patch 1: Use restart_syscall() instead of returning -ERESTARTNOINTR.
  This ensures the syscall is properly restarted without leaking the
  internal error code to userspace, fixing a test failure in
  tsync_test.competing_enablement. (Caught by Justin Suess, suggested
  by Tingmao Wang).
- Patch 1 and 2: Wrap comments to fit in 80 columns

Changes in v3:
- Patch 1: Changed down_write_killable() to down_write_trylock() and
  return -ERESTARTNOINTR on failure. This avoids a secondary deadlock 
  where a blocking wait prevents a sibling thread from waking up to 
  execute the requested TSYNC task_work. (Noted by Günther Noack. 
  down_write_interruptible() was also suggested but is not implemented 
  for rw_semaphores in the kernel).
- Patch 2: No changes.

Changes in v2:
- Split the changes into a 2-patch series.
- Patch 1: Adopted down_write_killable() instead of down_write().
- Patch 2: Removed wait_for_completion(&shared_ctx.all_prepared) and 
  replaced it with a `break` to prevent UAF.
  
Link to v3: https://lore.kernel.org/all/20260226015903.3158620-1-dingyihan@uniontech.com/
Link to v2: https://lore.kernel.org/all/20260225024734.3024732-1-dingyihan@uniontech.com/
Link to v1: https://lore.kernel.org/all/20260224062729.2908692-1-dingyihan@uniontech.com/

Yihan Ding (2):
  landlock: Serialize TSYNC thread restriction
  landlock: Clean up interrupted thread logic in TSYNC

 security/landlock/tsync.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

-- 
2.20.1