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

Justin Suess utilityemal77 at gmail.com
Tue Mar 3 18:13:17 UTC 2026 

On Tue, Mar 03, 2026 at 06:47:30PM +0100, Mickaël Salaün wrote:
> On Tue, Mar 03, 2026 at 11:20:10AM -0500, Justin Suess wrote:
> > 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 at syzkaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> > > Suggested-by: Günther Noack <gnoack3000 at gmail.com>
> > > Signed-off-by: Yihan Ding <dingyihan at 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
> 
> You're right, I have the same issue, not sure how I missed it last time.
> 
> > 
> > I have a fix that I will send as a patch.
> 
> I'll need to squash your fix to this fix to only have one non-buggy
> patch.  So, either you send a new patch and I'll squash it with
> Co-developed-by, or Yihan takes your patch and send a new version with
> your contribution (I'll prefer the later to make it easier to follow
> this series).
>
Agreed.

The latter option is probably better. Yihan, are you ok to squash / apply
my patch [1] yourself to your next version of this series?

Feel free to do whatever you think is best. (or submit your own fix if
you think mine isn't a good fit)

[1]: https://lore.kernel.org/linux-security-module/20260303174354.1839461-1-utilityemal77@gmail.com/
> > 
> > Kind Regards,
> > Justin Suess
> >

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