fix race between exit_itimers() and /proc/pid/timers

As Chris explains, the comment above exit_itimers() is not correct,
we can race with proc_timers_seq_ops. Change exit_itimers() to clear
signal->posix_timers with ->siglock held.

Cc: <stable@vger.kernel.org>
Reported-by: chris@accessvector.net
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/exec.c

@@ -1301,7 +1301,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	bprm->mm = NULL;
 
 #ifdef CONFIG_POSIX_TIMERS
-	exit_itimers(me->signal);
+	exit_itimers(me);
 	flush_itimer_signals();
 #endif
 

include/linux/sched/task.h

@@ -85,7 +85,7 @@ static inline void exit_thread(struct task_struct *tsk)
 extern __noreturn void do_group_exit(int);
 
 extern void exit_files(struct task_struct *);
-extern void exit_itimers(struct signal_struct *);
+extern void exit_itimers(struct task_struct *);
 
 extern pid_t kernel_clone(struct kernel_clone_args *kargs);
 struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node);

kernel/exit.c

@@ -766,7 +766,7 @@ void __noreturn do_exit(long code)
 
 #ifdef CONFIG_POSIX_TIMERS
 		hrtimer_cancel(&tsk->signal->real_timer);
-		exit_itimers(tsk->signal);
+		exit_itimers(tsk);
 #endif
 		if (tsk->mm)
 			setmax_mm_hiwater_rss(&tsk->signal->maxrss, tsk->mm);

kernel/time/posix-timers.c

@@ -1051,15 +1051,24 @@ static void itimer_delete(struct k_itimer *timer)
 }
 
 /*
- * This is called by do_exit or de_thread, only when there are no more
- * references to the shared signal_struct.
+ * This is called by do_exit or de_thread, only when nobody else can
+ * modify the signal->posix_timers list. Yet we need sighand->siglock
+ * to prevent the race with /proc/pid/timers.
  */
-void exit_itimers(struct signal_struct *sig)
+void exit_itimers(struct task_struct *tsk)
 {
+	struct list_head timers;
 	struct k_itimer *tmr;
 
-	while (!list_empty(&sig->posix_timers)) {
-		tmr = list_entry(sig->posix_timers.next, struct k_itimer, list);
+	if (list_empty(&tsk->signal->posix_timers))
+		return;
+
+	spin_lock_irq(&tsk->sighand->siglock);
+	list_replace_init(&tsk->signal->posix_timers, &timers);
+	spin_unlock_irq(&tsk->sighand->siglock);
+
+	while (!list_empty(&timers)) {
+		tmr = list_first_entry(&timers, struct k_itimer, list);
 		itimer_delete(tmr);
 	}
 }