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