fuse: fix use-after-free in fuse_direct_IO()

In async IO blocking case the additional reference to the io is taken for
it to survive fuse_aio_complete(). In non blocking case this additional
reference is not needed, however we still reference io to figure out
whether to wait for completion or not. This is wrong and will lead to
use-after-free. Fix it by storing blocking information in separate
variable.

This was spotted by KASAN when running generic/208 fstest.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 744742d692e3 ("fuse: Add reference counting for fuse_io_priv")
Cc: <stable@vger.kernel.org> # v4.6

fs/fuse/file.c

@@ -2924,10 +2924,12 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	}
 
 	if (io->async) {
+		bool blocking = io->blocking;
+
 		fuse_aio_complete(io, ret < 0 ? ret : 0, -1);
 
 		/* we have a non-extending, async request, so return */
-		if (!io->blocking)
+		if (!blocking)
 			return -EIOCBQUEUED;
 
 		wait_for_completion(&wait);