signalfd: fix interaction with posix-timers

dequeue_signal:

	if (__SI_TIMER) {
		spin_unlock(&tsk->sighand->siglock);
		do_schedule_next_timer(info);
		spin_lock(&tsk->sighand->siglock);
	}

Unless tsk == curent, this is absolutely unsafe: nothing prevents tsk from
exiting. If signalfd was passed to another process, do_schedule_next_timer()
is just wrong.

Add yet another "tsk == current" check into dequeue_signal().

This patch fixes an oopsable bug, but breaks the scheduling of posix timers
if the shared __SI_TIMER signal was fetched via signalfd attached to another
sub-thread. Mostly fixed by the next patch.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Michael Kerrisk <mtk-manpages@gmx.net>
Cc: Roland McGrath <roland@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/signal.c b/kernel/signal.c
index b27c01a..ad63109 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -378,7 +378,7 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
        /* We only dequeue private signals from ourselves, we don't let
         * signalfd steal them
         */
-       if (tsk == current)
+       if (likely(tsk == current))
                signr = __dequeue_signal(&tsk->pending, mask, info);
        if (!signr) {
                signr = __dequeue_signal(&tsk->signal->shared_pending,
@@ -425,7 +425,7 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
                if (!(tsk->signal->flags & SIGNAL_GROUP_EXIT))
                        tsk->signal->flags |= SIGNAL_STOP_DEQUEUED;
        }
-       if ( signr &&
+       if (signr && likely(tsk == current) &&
             ((info->si_code & __SI_MASK) == __SI_TIMER) &&
             info->si_sys_private){
                /*