[PATCH 5.18 015/231] fix race between exit_itimers() and /proc/pid/timers

Greg Kroah-Hartman
2022-07-19T12:49:06Z

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
@@ -1297,7 +1297,7 @@ 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
@@ -83,7 +83,7 @@ static inline void exit_thread(struct ta
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);
--- 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);
--- 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);
}
}