[PATCH] send_sigqueue: simplify and fix the race

send_sigqueue() checks PF_EXITING, then locks p->sighand->siglock.  This is
unsafe: 'p' can exit in between and set ->sighand = NULL.  The race is
theoretical, the window is tiny and irqs are disabled by the caller, so I
don't think we need the fix for -stable tree.

Convert send_sigqueue() to use lock_task_sighand() helper.

Also, delete 'p->flags & PF_EXITING' re-check, it is unneeded and the
comment is wrong.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 4 insertions, 37 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 0528a959daa9..4922928d91f6 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1309,12 +1309,10 @@ void sigqueue_free(struct sigqueue *q)
         __sigqueue_free(q);
 }
 
-int
-send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
+int send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
 {
         unsigned long flags;
         int ret = 0;
-        struct sighand_struct *sh;
 
         BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
 
@@ -1328,48 +1326,17 @@ send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
          */
         rcu_read_lock();
 
-        if (unlikely(p->flags & PF_EXITING)) {
+        if (!likely(lock_task_sighand(p, &flags))) {
                 ret = -1;
                 goto out_err;
         }
 
-retry:
-        sh = rcu_dereference(p->sighand);
-
-        spin_lock_irqsave(&sh->siglock, flags);
-        if (p->sighand != sh) {
-                /* We raced with exec() in a multithreaded process... */
-                spin_unlock_irqrestore(&sh->siglock, flags);
-                goto retry;
-        }
-
-        /*
-         * We do the check here again to handle the following scenario:
-         *
-         * CPU 0                CPU 1
-         * send_sigqueue
-         * check PF_EXITING
-         * interrupt            exit code running
-         *                      __exit_signal
-         *                      lock sighand->siglock
-         *                      unlock sighand->siglock
-         * lock sh->siglock
-         * add(tsk->pending)    flush_sigqueue(tsk->pending)
-         *
-         */
-
-        if (unlikely(p->flags & PF_EXITING)) {
-                ret = -1;
-                goto out;
-        }
-
         if (unlikely(!list_empty(&q->list))) {
                 /*
                  * If an SI_TIMER entry is already queue just increment
                  * the overrun count.
                  */
-                if (q->info.si_code != SI_TIMER)
-                        BUG();
+                BUG_ON(q->info.si_code != SI_TIMER);
                 q->info.si_overrun++;
                 goto out;
         }
@@ -1385,7 +1352,7 @@ retry:
                 signal_wake_up(p, sig == SIGKILL);
 
 out:
-        spin_unlock_irqrestore(&sh->siglock, flags);
+        unlock_task_sighand(p, &flags);
 out_err:
         rcu_read_unlock();