[PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Hyunwoo Kim]

A per-thread CPU timer holds a reference to the PID of the thread it is
attached to and, while it is armed, its node is queued in that thread's
posix_cputimers. The task is looked up by that PID.

When a non-leader thread exec()s, de_thread() changes which task owns
that PID. pid_task(timer->it.cpu.pid, PIDTYPE_PID) then returns NULL,
but the node is still queued on tsk, which is alive. timer_lock_sighand()
takes a failed lookup to mean that the node is already dequeued, so it
has nothing to undo.

begin_new_exec() calls posix_cpu_timers_exit(me) right after
exec_task_namespaces() and that removes the leftover node, so the state
normally stays invisible. But bprm->point_of_no_return is set before
de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or
exec_task_namespaces() fails, the task dies before it gets there.
exit_itimers() then frees the k_itimer while its node is still queued,
and reaping tsk later erases that freed node from the rbtree.

In short:

      the non-leader thread B           the parent

  timer_create(CLOCK_THREAD_CPUTIME_ID)
  timer_settime()
    arm_timer()            // the node is queued on B
  execve()
    de_thread(B)
      exchange_tids(B, leader)  // B's PID now belongs to the leader
      release_task(leader)
        __exit_signal(leader)
          posix_cpu_timers_exit(leader)  // cleans leader's queue, not B's
          __unhash_process(leader)  // that PID has no task anymore
    exec_mmap()
      mmap_read_lock_killable(old_mm)
                                kill(B, SIGKILL)
      // -EINTR
  get_signal()
    do_exit()
      exit_itimers()
        posix_timer_delete()
          posix_cpu_timer_del()
        posix_timer_unhash_and_free()  // freed while still queued
                                wait4()
                                  release_task(B)
                                    posix_cpu_timers_exit(B)
                                      cleanup_timerqueue()
                                        timerqueue_del()  // use-after-free

KASAN log:

  BUG: KASAN: slab-use-after-free in timerqueue_del+0x59/0x80
  Write of size 8 at addr ffff88801090e7e0 by task poc/99
  ...
  Call Trace:
   <TASK>
   timerqueue_del+0x59/0x80
   posix_cpu_timers_exit+0xa8/0xe0
   release_task+0x1c6/0xa40
   wait_consider_task+0x922/0x15f0
   __do_wait+0x3bc/0x3d0
   do_wait+0xba/0x1d0
   kernel_wait4+0xfe/0x1d0
   __do_sys_wait4+0x10b/0x120
   do_syscall_64+0xf9/0x540
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  ...
  Allocated by task 114:
   kmem_cache_alloc_noprof+0x143/0x3d0
   do_timer_create+0x156/0x890
   __x64_sys_timer_create+0x113/0x130
  ...
  Freed by task 104:
   __kasan_slab_free+0x43/0x70
   __rcu_free_sheaf_prepare+0x5c/0x260
   rcu_free_sheaf+0x1a/0xc0
   rcu_core+0x3d1/0xdf0
   handle_softirqs+0x133/0x400
  ...
  The buggy address belongs to the object at ffff88801090e700
   which belongs to the cache posix_timers_cache of size 384
  The buggy address is located 224 bytes inside of
   freed 384-byte region [ffff88801090e700, ffff88801090e880)

Dequeue the per-thread CPU timers of tsk before the PID changes hands, so
that a failed lookup again implies a dequeued node. Process-wide timers
are looked up with PIDTYPE_TGID and transfer_pid() moves that link to
tsk, so they are left alone.

Fixes: 55e8c8eb2c7b ("posix-cpu-timers: Store a reference to a pid not a task")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
Changes in v2:
- Add the trigger sequence and the KASAN log to the commit message.
- v1: https://lore.kernel.org/all/anfgrsPlUdwBhdrp@v4bel/
---
 fs/exec.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 745f6eb5279e6..9b2d140bd9f2a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1003,6 +1003,18 @@ static int de_thread(struct task_struct *tsk)
 		 * the former thread group leader:
 		 */
 
+#ifdef CONFIG_POSIX_TIMERS
+		/*
+		 * exchange_tids() hands this thread's PID to the old leader,
+		 * which is reaped right after. The PID lookup in
+		 * timer_lock_sighand() then fails while the per thread CPU
+		 * timers are still queued here, so dequeue them first.
+		 */
+		spin_lock(lock);
+		posix_cpu_timers_exit(tsk);
+		spin_unlock(lock);
+#endif
+
 		/* Become a process group leader with the old leader's pid.
 		 * The old leader becomes a thread of the this thread group.
 		 */
-- 
2.43.0

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Oleg Nesterov]

OMG, my head spins ;) I'll try to read your explanation tomorrow.
Right now I am all confused.

But let me ask some stupid questions right now, please help me to
understand the problem.

On 08/26, Hyunwoo Kim wrote:
>
> begin_new_exec() calls posix_cpu_timers_exit(me) right after
> exec_task_namespaces() and that removes the leftover node, so the state
> normally stays invisible. But bprm->point_of_no_return is set before
> de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or
> exec_task_namespaces() fails, the task dies before it gets there.

Do I understand this correctly? If begin_new_exec() does call
posix_cpu_timers_exit(me), then everything is fine.

Yes? If yes

> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1003,6 +1003,18 @@ static int de_thread(struct task_struct *tsk)
>  		 * the former thread group leader:
>  		 */
>
> +#ifdef CONFIG_POSIX_TIMERS
> +		/*
> +		 * exchange_tids() hands this thread's PID to the old leader,
> +		 * which is reaped right after. The PID lookup in
> +		 * timer_lock_sighand() then fails while the per thread CPU
> +		 * timers are still queued here, so dequeue them first.
> +		 */
> +		spin_lock(lock);
> +		posix_cpu_timers_exit(tsk);
> +		spin_unlock(lock);
> +#endif

... then why do we need to call posix_cpu_timers_exit(current) before
exchange_tids() ?

And what if another process attaches another cpu timer to the execing
thread right after the code above?

Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
sequence in begin_new_exec() up, right after de_thread() before the
"if (retval)" check?

Thank you,

Oleg.

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Hyunwoo Kim]

On Wed, Aug 26, 2026 at 02:55:31PM +0200, Oleg Nesterov wrote:
> OMG, my head spins ;) I'll try to read your explanation tomorrow.
> Right now I am all confused.
> 
> But let me ask some stupid questions right now, please help me to
> understand the problem.
> 
> On 08/26, Hyunwoo Kim wrote:
> >
> > begin_new_exec() calls posix_cpu_timers_exit(me) right after
> > exec_task_namespaces() and that removes the leftover node, so the state
> > normally stays invisible. But bprm->point_of_no_return is set before
> > de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or
> > exec_task_namespaces() fails, the task dies before it gets there.
> 
> Do I understand this correctly? If begin_new_exec() does call
> posix_cpu_timers_exit(me), then everything is fine.

Yes. A failure after it, e.g. unshare_sighand(), is harmless.

> 
> Yes? If yes
> 
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1003,6 +1003,18 @@ static int de_thread(struct task_struct *tsk)
> >  		 * the former thread group leader:
> >  		 */
> >
> > +#ifdef CONFIG_POSIX_TIMERS
> > +		/*
> > +		 * exchange_tids() hands this thread's PID to the old leader,
> > +		 * which is reaped right after. The PID lookup in
> > +		 * timer_lock_sighand() then fails while the per thread CPU
> > +		 * timers are still queued here, so dequeue them first.
> > +		 */
> > +		spin_lock(lock);
> > +		posix_cpu_timers_exit(tsk);
> > +		spin_unlock(lock);
> > +#endif
> 
> ... then why do we need to call posix_cpu_timers_exit(current) before
> exchange_tids() ?

Strictly we don't. posix_cpu_timers_exit() walks tsk->posix_cputimers
directly and never does the PID lookup, so it only has to happen before
exit_itimers()

I put it before exchange_tids() so that the state timer_lock_sighand()
warns about never exists in the first place.

> 
> And what if another process attaches another cpu timer to the execing
> thread right after the code above?

Not for a per-thread timer, pid_for_clock() requires same_thread_group()
and there is no other thread left at that point.

A process wide timer has no such check, but it goes to
signal->posix_cputimers and the TGID lookup stays valid, so it isn't
affected.

> 
> Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
> sequence in begin_new_exec() up, right after de_thread() before the
> "if (retval)" check?

posix_cpu_timers_exit(me) there is enough. I haven't tested moving the 
whole block.


Best regards,
Hyunwoo Kim

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Thomas Gleixner]

On Wed, Aug 26 2026 at 20:41, Hyunwoo Kim wrote:
> diff --git a/fs/exec.c b/fs/exec.c
> index 745f6eb5279e6..9b2d140bd9f2a 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1003,6 +1003,18 @@ static int de_thread(struct task_struct *tsk)
>  		 * the former thread group leader:
>  		 */
>  
> +#ifdef CONFIG_POSIX_TIMERS
> +		/*
> +		 * exchange_tids() hands this thread's PID to the old leader,
> +		 * which is reaped right after. The PID lookup in
> +		 * timer_lock_sighand() then fails while the per thread CPU
> +		 * timers are still queued here, so dequeue them first.
> +		 */
> +		spin_lock(lock);
> +		posix_cpu_timers_exit(tsk);
> +		spin_unlock(lock);
> +#endif
> +
>  		/* Become a process group leader with the old leader's pid.
>  		 * The old leader becomes a thread of the this thread group.
>  		 */

Similar to the other issue vs. POSIX timer signals this really wants to
be solved in exit() independent of non-leader exec(). Let me have a
look.

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Thomas Gleixner]

On Mon, Aug 31 2026 at 13:56, Thomas Gleixner wrote:
> On Wed, Aug 26 2026 at 20:41, Hyunwoo Kim wrote:
>> diff --git a/fs/exec.c b/fs/exec.c
>> index 745f6eb5279e6..9b2d140bd9f2a 100644
>> --- a/fs/exec.c
>> +++ b/fs/exec.c
>> @@ -1003,6 +1003,18 @@ static int de_thread(struct task_struct *tsk)
>>  		 * the former thread group leader:
>>  		 */
>>  
>> +#ifdef CONFIG_POSIX_TIMERS
>> +		/*
>> +		 * exchange_tids() hands this thread's PID to the old leader,
>> +		 * which is reaped right after. The PID lookup in
>> +		 * timer_lock_sighand() then fails while the per thread CPU
>> +		 * timers are still queued here, so dequeue them first.
>> +		 */
>> +		spin_lock(lock);
>> +		posix_cpu_timers_exit(tsk);
>> +		spin_unlock(lock);
>> +#endif
>> +
>>  		/* Become a process group leader with the old leader's pid.
>>  		 * The old leader becomes a thread of the this thread group.
>>  		 */
>
> Similar to the other issue vs. POSIX timer signals this really wants to
> be solved in exit() independent of non-leader exec(). Let me have a
> look.

Duh. That's about the non-leader itself....

Why did anyone think that swapping TIDs is a brilliant idea?

I still think that this should be done differently. There is no point in
doing the disarming in release_task(). Let me think some more.

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Thomas Gleixner]

On Wed, Aug 26 2026 at 14:55, Oleg Nesterov wrote:
> Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
> sequence in begin_new_exec() up, right after de_thread() before the
> "if (retval)" check?

Yes, but that does not buy us anything.

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Oleg Nesterov]

On 08/31, Thomas Gleixner wrote:
>
> On Wed, Aug 26 2026 at 14:55, Oleg Nesterov wrote:
> > Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
> > sequence in begin_new_exec() up, right after de_thread() before the
> > "if (retval)" check?
>
> Yes, but that does not buy us anything.

Hmm. I don't understand but this doesn't matter.

I didn't mean this makes sense, I just tried to understand the Hyunwoo's
fix, in particular the placement of additional posix_cpu_timers_exit().

Oleg.

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Thomas Gleixner]

On Mon, Aug 31 2026 at 20:24, Oleg Nesterov wrote:
> On 08/31, Thomas Gleixner wrote:
>>
>> On Wed, Aug 26 2026 at 14:55, Oleg Nesterov wrote:
>> > Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
>> > sequence in begin_new_exec() up, right after de_thread() before the
>> > "if (retval)" check?
>>
>> Yes, but that does not buy us anything.
>
> I didn't mean this makes sense, I just tried to understand the Hyunwoo's
> fix, in particular the placement of additional posix_cpu_timers_exit().

Fair enough. But as I was staring at it some more, I thought more about
it. Yes, we can do:

     ret = de_thread()
     if (ret)
     	goto out;

     mop_up_posix_timers();

de_thread() only returns failure _before_ the TID swap, so that's safe
and spares sprinkling this extra invocation into de_thread().

What a mess.

Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before exchange_tids()

[Thomas Gleixner]

On Tue, Sep 01 2026 at 16:50, Thomas Gleixner wrote:
> On Mon, Aug 31 2026 at 20:24, Oleg Nesterov wrote:
>> On 08/31, Thomas Gleixner wrote:
>>>
>>> On Wed, Aug 26 2026 at 14:55, Oleg Nesterov wrote:
>>> > Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
>>> > sequence in begin_new_exec() up, right after de_thread() before the
>>> > "if (retval)" check?
>>>
>>> Yes, but that does not buy us anything.
>>
>> I didn't mean this makes sense, I just tried to understand the Hyunwoo's
>> fix, in particular the placement of additional posix_cpu_timers_exit().
>
> Fair enough. But as I was staring at it some more, I thought more about
> it. Yes, we can do:
>
>      ret = de_thread()
>      if (ret)
>      	goto out;
>
>      mop_up_posix_timers();
>
> de_thread() only returns failure _before_ the TID swap, so that's safe
> and spares sprinkling this extra invocation into de_thread().
>
> What a mess.

I'm still thinking that an indicator for a task doing exec(), which is
set _before_ de_thread() and removed afterwards would be more sensible
than relying on subtle ordering constraints.

Along with the nonsense to keep timers queued on a task until
release_task(). There is no real benefit from keeping them around at
after PF_EXITING is set. Sure someone might argue that the precious CPU
cycles consumed in do_exit() afterwards won't be accounted, but
seriously?

Thanks,

        tglx