fuse: Verify userspace asks to requeue interrupt that we really sent

When queue_interrupt() is called from fuse_dev_do_write(), it came from
userspace directly. Userspace may pass any request id, even the request's
we have not interrupted (or even background's request). This patch adds
sanity check to make kernel safe against that.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dev.c

@@ -479,9 +479,15 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
 	fuse_put_request(fc, req);
 }
 
-static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
+static int queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
 {
 	spin_lock(&fiq->waitq.lock);
+	/* Check for we've sent request to interrupt this req */
+	if (unlikely(!test_bit(FR_INTERRUPTED, &req->flags))) {
+		spin_unlock(&fiq->waitq.lock);
+		return -EINVAL;
+	}
+
 	if (list_empty(&req->intr_entry)) {
 		list_add_tail(&req->intr_entry, &fiq->interrupts);
 		/*
@@ -492,12 +498,13 @@ static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
 		if (test_bit(FR_FINISHED, &req->flags)) {
 			list_del_init(&req->intr_entry);
 			spin_unlock(&fiq->waitq.lock);
-			return;
+			return 0;
 		}
 		wake_up_locked(&fiq->waitq);
 		kill_fasync(&fiq->fasync, SIGIO, POLL_IN);
 	}
 	spin_unlock(&fiq->waitq.lock);
+	return 0;
 }
 
 static void request_wait_answer(struct fuse_conn *fc, struct fuse_req *req)
@@ -1962,7 +1969,7 @@ static ssize_t fuse_dev_do_write(struct fuse_dev *fud,
 		else if (oh.error == -ENOSYS)
 			fc->no_interrupt = 1;
 		else if (oh.error == -EAGAIN)
-			queue_interrupt(&fc->iq, req);
+			err = queue_interrupt(&fc->iq, req);
 
 		fuse_put_request(fc, req);