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

[Thomas Gleixner <tglx@kernel.org>]

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
---
Changes in v3:
  Move the cleanup right after de_thread() - Oleg
  Rework change log

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 |   29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1115,6 +1115,17 @@ static struct file *bprm_identity_file(c
 	return bprm->file;
 }
 
+static void posixtimer_exec(struct task_struct *me)
+{
+#ifdef CONFIG_POSIX_TIMERS
+	spin_lock_irq(&me->sighand->siglock);
+	posix_cpu_timers_exit(me);
+	spin_unlock_irq(&me->sighand->siglock);
+	exit_itimers(me);
+	flush_itimer_signals();
+#endif
+}
+
 /*
  * Calling this is the point of no return. None of the failures will be
  * seen by userspace since either the process is already taking a fatal
@@ -1152,6 +1163,16 @@ int begin_new_exec(struct linux_binprm *
 	retval = de_thread(me);
 	if (retval)
 		goto out;
+
+	/*
+	 * This must be done here to ensure that POSIX CPU timers which were
+	 * armed on the current task are dequeued from me::posix_cputimers.
+	 * That ensures that in case of a TID switch the deletion of the related
+	 * POSIX timer will not free an enqueued timer because the TID lookup
+	 * failed as the original target TID was the old leader.
+	 */
+	posixtimer_exec(me);
+
 	/* see the comment in check_unsafe_exec() */
 	current->fs->in_exec = 0;
 	/*
@@ -1192,14 +1213,6 @@ int begin_new_exec(struct linux_binprm *
 	if (retval)
 		goto out_unlock;
 
-#ifdef CONFIG_POSIX_TIMERS
-	spin_lock_irq(&me->sighand->siglock);
-	posix_cpu_timers_exit(me);
-	spin_unlock_irq(&me->sighand->siglock);
-	exit_itimers(me);
-	flush_itimer_signals();
-#endif
-
 	/*
 	 * Make the signal table private.
 	 */