CVE-2022-2585 - Linux kernel POSIX CPU timer UAF

It was discovered that when exec'ing from a non-leader thread, armed POSIX
CPU timers would be left on a list but freed, leading to a use-after-free.

An independent security researcher working with SSD Secure Disclosure
discovered that this vulnerability could be exploited for Local Privilege
Escalation.

This bug was introduced by commit 55e8c8eb2c7b ("posix-cpu-timers: Store a
reference to a pid not a task"), which is present since v5.7-rc1.

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/livepatch.h>

static int disable_posix_cpu_timer_create(void)
{
	dump_stack();
	return -ENOSYS;
}

static struct klp_func funcs[] = {
	{
		.old_name = "posix_cpu_timer_create",
		.new_func = disable_posix_cpu_timer_create,
	}, { }
};

static struct klp_object objs[] = {
	{
		.funcs = funcs,
	}, { }
};

static struct klp_patch patch = {
	.mod = THIS_MODULE,
	.objs = objs,
};

static int livepatch_init(void)
{
	return klp_enable_patch(&patch);

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