From 9e74142ab2fe94f2b12626357cb2cbe7075b9b4f Mon Sep 17 00:00:00 2001
From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Date: Fri, 22 Jul 2022 06:52:11 -0300
Subject: [PATCH] posix-cpu-timers: Cleanup CPU timers before freeing them
 during exec
Patch-mainline: Not yet, embargoed
References: CVE-2022-2585 bsc#1202094

Commit 55e8c8eb2c7b ("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: 55e8c8eb2c7b ("posix-cpu-timers: Store a reference to a pid not a task")
Reported-by: "An independent security researcher working with SSD Secure Disclosure"
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Frederic Weisbecker <fweisbecker@suse.de>
---
 fs/exec.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 778123259e42..1c6b477dad69 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1301,6 +1301,9 @@ 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
-- 
2.34.1