posix-cpu-timers: Cleanup CPU timers before freeing them during exec

commit af5644453e8833d9af643205ee20d42635114cb3 upstream.

Commit 52a323cd661b ("posix-cpu-timers: Store a reference to a pid not a
task") started looking up tasks by PID when deleting a CPU timer.

When a non-leader thread calls execve, it will switch PIDs with the leader
process. Then, as it calls exit_itimers, posix_cpu_timer_del cannot find
the task because the timer still points out to the old PID.

That means that armed timers won't be disarmed, that is, they won't be
removed from the timerqueue_list. exit_itimers will still release their
memory, and when that list is later processed, it leads to a
use-after-free.

Clean up the timers from the de-threaded task before freeing them. This
prevents a reported use-after-free.

Fixes: 52a323cd661b ("posix-cpu-timers: Store a reference to a pid not a task")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220809170751.164716-1-cascardo@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/exec.c b/fs/exec.c
index 76196ddefbdd0cf8b88b426c6b3938b8d0535aab..7d424337b4ec927eb17bfd8d2351b79418d50371 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1298,6 +1298,9 @@ int begin_new_exec(struct linux_binprm * bprm)
        bprm->mm = NULL;
 
 #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