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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, chris@accessvector.net,
        Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 5.10 008/112] fix race between exit_itimers() and /proc/pid/timers
Date:   Tue, 19 Jul 2022 13:53:01 +0200
Message-Id: <20220719114626.799742622@linuxfoundation.org>
In-Reply-To: <20220719114626.156073229@linuxfoundation.org>
References: <20220719114626.156073229@linuxfoundation.org>
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From: Oleg Nesterov <oleg@redhat.com>

commit d5b36a4dbd06c5e8e36ca8ccc552f679069e2946 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 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(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1286,7 +1286,7 @@ int begin_new_exec(struct linux_binprm *
 	bprm->mm = NULL;
 
 #ifdef CONFIG_POSIX_TIMERS
-	exit_itimers(me->signal);
+	exit_itimers(me);
 	flush_itimer_signals();
 #endif
 
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -82,7 +82,7 @@ static inline void exit_thread(struct ta
 extern 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 *fork_idle(int);
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -782,7 +782,7 @@ 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);
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1051,15 +1051,24 @@ retry_delete:
 }
 
 /*
- * 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);
 	}
 }