fuse: Set *nbytesp=0 in fuse_get_user_pages on allocation failure

In fuse_get_user_pages(), set *nbytesp to 0 when struct page **pages
allocation fails. This prevents the caller (fuse_direct_io) from making
incorrect assumptions that could lead to NULL pointer dereferences
when processing the request reply.

Previously, *nbytesp was left unmodified on allocation failure, which
could cause issues if the caller assumed pages had been added to
ap->descs[] when they hadn't.

Reported-by: syzbot+87b8e6ed25dbc41759f7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=87b8e6ed25dbc41759f7
Fixes: 3b97c36 ("fuse: convert direct io to use folios")
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Tested-by: Dmitry Antipov <dmantipov@yandex.ru>
Tested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: bsbernd

fs/fuse/file.c

@@ -1541,8 +1541,10 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
 	 */
 	struct page **pages = kzalloc(max_pages * sizeof(struct page *),
 				      GFP_KERNEL);
-	if (!pages)
-		return -ENOMEM;
+	if (!pages) {
+		ret = -ENOMEM;
+		goto out;
+	}
 
 	while (nbytes < *nbytesp && nr_pages < max_pages) {
 		unsigned nfolios, i;
@@ -1588,6 +1590,7 @@ static int fuse_get_user_pages(struct fuse_args_pages *ap, struct iov_iter *ii,
 	else
 		ap->args.out_pages = true;
 
+out:
 	*nbytesp = nbytes;
 
 	return ret < 0 ? ret : 0;