Re: [PATCH v3 1/2] landlock: Serialize TSYNC thread restriction

[Justin Suess <utilityemal77@gmail.com>]

On Thu, Feb 26, 2026 at 09:59:02AM +0800, Yihan Ding wrote:
> syzbot found a deadlock in landlock_restrict_sibling_threads().
> When multiple threads concurrently call landlock_restrict_self() with
> sibling thread restriction enabled, they can deadlock by mutually
> queueing task_works on each other and then blocking in kernel space
> (waiting for the other to finish).
> 
> Fix this by serializing the TSYNC operations within the same process
> using the exec_update_lock. This prevents concurrent invocations
> from deadlocking. 
> 
> We use down_write_trylock() and return -ERESTARTNOINTR if the lock
> cannot be acquired immediately. This ensures that if a thread fails
> to get the lock, it will return to userspace, allowing it to process
> any pending TSYNC task_works from the lock holder, and then
> transparently restart the syscall.
> 
> Fixes: 42fc7e6543f6 ("landlock: Multithreading support for landlock_restrict_self()")
> Reported-by: syzbot+7ea2f5e9dfd468201817@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> Suggested-by: Günther Noack <gnoack3000@gmail.com>
> Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
> ---
> Changes in v3:
> - Replaced down_write_killable() with down_write_trylock() and 
>   returned -ERESTARTNOINTR to avoid a secondary deadlock caused by 
>   blocking the execution of task_works. (Caught by Günther Noack).
> 
> Changes in v2:
> - Use down_write_killable() instead of down_write().
> - Split the interrupt path cleanup into a separate patch.
> ---
>  security/landlock/tsync.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
> index de01aa899751..xxxxxxxxxxxx 100644
> --- a/security/landlock/tsync.c
> +++ b/security/landlock/tsync.c
> @@ -447,6 +447,13 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
>  	shared_ctx.new_cred = new_cred;
>  	shared_ctx.set_no_new_privs = task_no_new_privs(current);
>  
> +	/*
> +	 * Serialize concurrent TSYNC operations to prevent deadlocks
> +	 * when multiple threads call landlock_restrict_self() simultaneously.
> +	 */
> +	if (!down_write_trylock(&current->signal->exec_update_lock))
> +		return -ERESTARTNOINTR;
These two lines above introduced a test failure in tsync_test
completing_enablement.

The commit that introduced the bug is 3d6327c306b3e1356ab868bf27a0854669295a4f
(this patch) and is currently in the mic/next branch.

I noticed the test failure while testing an unrelated patch.

The bug is because this code never actually yields or restarts the syscall.

This is the test output I observed:

  [+] Running tsync_test:
  TAP version 13
  1..4
  # Starting 4 tests from 1 test cases.
  #  RUN           global.single_threaded_success ...
  #            OK  global.single_threaded_success
  ok 1 global.single_threaded_success
  #  RUN           global.multi_threaded_success ...
  #            OK  global.multi_threaded_success
  ok 2 global.multi_threaded_success
  #  RUN           global.multi_threaded_success_despite_diverging_domains ...
  #            OK  global.multi_threaded_success_despite_diverging_domains
  ok 3 global.multi_threaded_success_despite_diverging_domains
  #  RUN           global.competing_enablement ...
  # tsync_test.c:156:competing_enablement:Expected 0 (0) == d[1].result (-1)
  # competing_enablement: Test failed
  #          FAIL  global.competing_enablement
  not ok 4 global.competing_enablement
  # FAILED: 3 / 4 tests passed.


Brief investigation and the additions of these pr_warn lines:


diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 0d66a68677b7..84909232b220 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -574,6 +574,9 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 		const int err = landlock_restrict_sibling_threads(
 			current_cred(), new_cred);
 		if (err) {
+			pr_warn("landlock: restrict_self tsync err pid=%d tgid=%d err=%d flags=0x%x ruleset_fd=%d\n",
+				task_pid_nr(current), task_tgid_nr(current), err,
+				flags, ruleset_fd);
 			abort_creds(new_cred);
 			return err;
 		}
diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
index 5afc5d639b8f..deb0f0b1f081 100644
--- a/security/landlock/tsync.c
+++ b/security/landlock/tsync.c
@@ -489,8 +489,11 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
 	 * Serialize concurrent TSYNC operations to prevent deadlocks when multiple
 	 * threads call landlock_restrict_self() simultaneously.
 	 */
-	if (!down_write_trylock(&current->signal->exec_update_lock))
+	if (!down_write_trylock(&current->signal->exec_update_lock)) {
+		pr_warn("landlock: tsync trylock busy pid=%d tgid=%d\n",
+			task_pid_nr(current), task_tgid_nr(current));
 		return -ERESTARTNOINTR;
+	}
 
 	/*
 	 * We schedule a pseudo-signal task_work for each of the calling task's
@@ -602,6 +605,10 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
 
 	tsync_works_release(&works);
 	up_write(&current->signal->exec_update_lock);
+	if (atomic_read(&shared_ctx.preparation_error))
+		pr_warn("landlock: tsync preparation_error pid=%d tgid=%d err=%d\n",
+			task_pid_nr(current), task_tgid_nr(current),
+			atomic_read(&shared_ctx.preparation_error));
 
 	return atomic_read(&shared_ctx.preparation_error);
 }

Resulted in the following output:

  landlock: tsync trylock busy pid=1263 tgid=1261
  landlock: landlock: restrict_self tsync err pid=1263 tgid=1261 err=-513 flags=0x8 ruleset_fd=6
  # tsync_test.c:156:competing_enablement:Expected 0 (0) == d[1].result (-1)
  # competing_enablement: Test failed
  #          FAIL  global.competing_enablement
  not ok 4 global.competing_enablement

I have a fix that I will send as a patch.

Kind Regards,
Justin Suess