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>
---
 fs/exec.c                  |  2 +-
 include/linux/sched/task.h |  2 +-
 kernel/exit.c              |  2 +-
 kernel/time/posix-timers.c | 19 ++++++++++++++-----
 4 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 0989fb8472a1..778123259e42 100644
--- a/fs/exec.c
+++ b/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
 
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 505aaf9fe477..81cab4b01edc 100644
--- a/include/linux/sched/task.h
+++ b/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);
diff --git a/kernel/exit.c b/kernel/exit.c
index f072959fcab7..64c938ce36fe 100644
--- a/kernel/exit.c
+++ b/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);
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 1cd10b102c51..5dead89308b7 100644
--- a/kernel/time/posix-timers.c
+++ b/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);
 	}
 }
-- 
2.25.1.362.g51ebf55

On Mon, Jul 11, 2022 at 9:16 AM Oleg Nesterov <oleg@redhat.com> wrote:
>
> 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.

Thanks. Applied,

                  Linus