Re: [patch V2 2/8] exec: Cleanup POSIX timers right after de_thread()

[Frederic Weisbecker <frederic@kernel.org>]

Le Sat, Sep 05, 2026 at 08:59:06PM +0200, Thomas Gleixner a écrit :
> From: Hyunwoo Kim <imv4bel@gmail.com>
> 
> 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
> 
> Move the POSIX timer cleanup right after de_thread() before any of the
> later failure conditions brings the task into do_exit().
> 
> [ tglx: Move the cleanup right after de_thread() ]
> 
> Fixes: 55e8c8eb2c7b ("posix-cpu-timers: Store a reference to a pid not a task")
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> Cc: stable@vger.kernel.org
> Link: https://patch.msgid.link/ao7Q8miiuLAPVnWv@v4bel

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>