[PATCH] posix-cpu-timers: Fix pid refcount leak in do_cpu_nanosleep() signal path

["Cen Zhang (Microsoft)" <cenzhang@linux.microsoft.com>]

do_cpu_nanosleep() sleeps on a temporary k_itimer whose reference on the
target's struct pid is taken by posix_cpu_timer_create() and dropped by
posix_cpu_timer_del(). When a signal ends the sleep, the timer is
disarmed with posix_cpu_timer_set(), but posix_cpu_timer_del() is only
called for a result of 0 or TIMER_RETRY. If the target thread or process
has exited in the meantime, timer_lock_sighand() finds no task,
posix_cpu_timer_set() returns -ESRCH and the pid reference is leaked.

An unprivileged user can trigger this: sleep on the CPU clock of a
thread, let that thread exit, then signal the sleeper. Each round leaks
one struct pid. Repeating it exhausts unreclaimable slab memory
within minutes:

  Out of memory: Killed process 150 (poc) total-vm:789292kB, anon-rss:484kB
  Kernel panic - not syncing: System is deadlocked on memory

Fix this by also calling posix_cpu_timer_del() on -ESRCH. With the
target gone, the deletion is safe. It finds no task either and only
drops the pid reference.

Fixes: e26d70d271ee ("posix-timers: Cleanup reaped target handling")
Reported-by: AutonomousCodeSecurity@microsoft.com
Cc: stable@vger.kernel.org
Signed-off-by: Cen Zhang (Microsoft) <cenzhang@linux.microsoft.com>
---
 kernel/time/posix-cpu-timers.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 0bf4fcd969c8..65cc612046d1 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1600,8 +1600,8 @@ static int do_cpu_nanosleep(const clockid_t which_clock, int flags,
 		 */
 		expires = cpu_timer_getexpires(&timer.it.cpu);
 		error = posix_cpu_timer_set(&timer, 0, &zero_it, &it);
-		if (!error) {
-			/* Timer is now unarmed, deletion can not fail. */
+		if (error != TIMER_RETRY) {
+			/* Unarmed or target gone (-ESRCH), deletion can not fail. */
 			posix_cpu_timer_del(&timer);
 		} else {
 			while (error == TIMER_RETRY) {
-- 
2.55.0