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

[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:

  de_thread(tsk)
    exchange_tids(tsk, leader);		// tsk's PID now belongs to leader
    ...
    release_task(leader)
      __exit_signal(leader)
        posix_cpu_timers_exit(leader);	// cleans leader's queue, not tsk's
        __unhash_process(leader)	// that PID has no task anymore

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.

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>
---
 fs/exec.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index c7b8f2d6366c44..f80f70e1c26de4 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1000,6 +1000,18 @@ 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