1 files changed, 39 insertions, 7 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 506c8491a8d1..40a8f462a822 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1256,6 +1256,33 @@ out:
 	return un;
 }
 
+
+/**
+ * get_queue_result - Retrieve the result code from sem_queue
+ * @q: Pointer to queue structure
+ *
+ * Retrieve the return code from the pending queue. If IN_WAKEUP is found in
+ * q->status, then we must loop until the value is replaced with the final
+ * value: This may happen if a task is woken up by an unrelated event (e.g.
+ * signal) and in parallel the task is woken up by another task because it got
+ * the requested semaphores.
+ *
+ * The function can be called with or without holding the semaphore spinlock.
+ */
+static int get_queue_result(struct sem_queue *q)
+{
+	int error;
+
+	error = q->status;
+	while (unlikely(error == IN_WAKEUP)) {
+		cpu_relax();
+		error = q->status;
+	}
+
+	return error;
+}
+
+
 SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 		unsigned, nsops, const struct timespec __user *, timeout)
 {
@@ -1409,15 +1436,18 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 	else
 		schedule();
 
-	error = queue.status;
-	while(unlikely(error == IN_WAKEUP)) {
-		cpu_relax();
-		error = queue.status;
-	}
+	error = get_queue_result(&queue);
 
 	if (error != -EINTR) {
 		/* fast path: update_queue already obtained all requested
-		 * resources */
+		 * resources.
+		 * Perform a smp_mb(): User space could assume that semop()
+		 * is a memory barrier: Without the mb(), the cpu could
+		 * speculatively read in user space stale data that was
+		 * overwritten by the previous owner of the semaphore.
+		 */
+		smp_mb();
+
 		goto out_free;
 	}
 
@@ -1427,10 +1457,12 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 		goto out_free;
 	}
 
+	error = get_queue_result(&queue);
+
 	/*
 	 * If queue.status != -EINTR we are woken up by another process
 	 */
-	error = queue.status;
+
 	if (error != -EINTR) {
 		goto out_unlock_free;
 	}