[PATCH] fix send_sigqueue() vs thread exit race

posix_timer_event() first checks that the thread (SIGEV_THREAD_ID case)
does not have PF_EXITING flag, then it calls send_sigqueue() which locks
task list.  But if the thread exits in between the kernel will oops
(->sighand == NULL after __exit_sighand).

This patch moves the PF_EXITING check into the send_sigqueue(), it must be
done atomically under tasklist_lock.  When send_sigqueue() detects exiting
thread it returns -1.  In that case posix_timer_event will send the signal
to thread group.

Also, this patch fixes task_struct use-after-free in posix_timer_event.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/posix-timers.c

@@ -427,21 +427,23 @@ int posix_timer_event(struct k_itimer *timr,int si_private)
 	timr->sigq->info.si_code = SI_TIMER;
 	timr->sigq->info.si_tid = timr->it_id;
 	timr->sigq->info.si_value = timr->it_sigev_value;
+
 	if (timr->it_sigev_notify & SIGEV_THREAD_ID) {
-		if (unlikely(timr->it_process->flags & PF_EXITING)) {
-			timr->it_sigev_notify = SIGEV_SIGNAL;
-			put_task_struct(timr->it_process);
-			timr->it_process = timr->it_process->group_leader;
-			goto group;
-		}
-		return send_sigqueue(timr->it_sigev_signo, timr->sigq,
-			timr->it_process);
-	}
-	else {
-	group:
-		return send_group_sigqueue(timr->it_sigev_signo, timr->sigq,
-			timr->it_process);
+		struct task_struct *leader;
+		int ret = send_sigqueue(timr->it_sigev_signo, timr->sigq,
+					timr->it_process);
+
+		if (likely(ret >= 0))
+			return ret;
+
+		timr->it_sigev_notify = SIGEV_SIGNAL;
+		leader = timr->it_process->group_leader;
+		put_task_struct(timr->it_process);
+		timr->it_process = leader;
 	}
+
+	return send_group_sigqueue(timr->it_sigev_signo, timr->sigq,
+				   timr->it_process);
 }
 EXPORT_SYMBOL_GPL(posix_timer_event);
 

kernel/signal.c

@@ -1366,16 +1366,16 @@ send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
 	unsigned long flags;
 	int ret = 0;
 
-	/*
-	 * We need the tasklist lock even for the specific
-	 * thread case (when we don't need to follow the group
-	 * lists) in order to avoid races with "p->sighand"
-	 * going away or changing from under us.
-	 */
 	BUG_ON(!(q->flags & SIGQUEUE_PREALLOC));
-	read_lock(&tasklist_lock);  
+	read_lock(&tasklist_lock);
+
+	if (unlikely(p->flags & PF_EXITING)) {
+		ret = -1;
+		goto out_err;
+	}
+
 	spin_lock_irqsave(&p->sighand->siglock, flags);
-	
+
 	if (unlikely(!list_empty(&q->list))) {
 		/*
 		 * If an SI_TIMER entry is already queue just increment
@@ -1385,7 +1385,7 @@ send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
 			BUG();
 		q->info.si_overrun++;
 		goto out;
-	} 
+	}
 	/* Short-circuit ignored signals.  */
 	if (sig_ignored(p, sig)) {
 		ret = 1;
@@ -1400,8 +1400,10 @@ send_sigqueue(int sig, struct sigqueue *q, struct task_struct *p)
 
 out:
 	spin_unlock_irqrestore(&p->sighand->siglock, flags);
+out_err:
 	read_unlock(&tasklist_lock);
-	return(ret);
+
+	return ret;
 }
 
 int