fanotify: sanitize handle_type values when reporting fid

Unlike file_handle, type and len of struct fanotify_fh are u8.
Traditionally, filesystem return handle_type < 0xff, but there
is no enforecement for that in vfs.

Add a sanity check in fanotify to avoid truncating handle_type
if its value is > 0xff.

Fixes: 7cdafe6 ("exportfs: check for error return value from exportfs_encode_*()")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20250627104835.184495-1-amir73il@gmail.com

Author: amir73il

fs/notify/fanotify/fanotify.c

@@ -454,7 +454,13 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
 	dwords = fh_len >> 2;
 	type = exportfs_encode_fid(inode, buf, &dwords);
 	err = -EINVAL;
-	if (type <= 0 || type == FILEID_INVALID || fh_len != dwords << 2)
+	/*
+	 * Unlike file_handle, type and len of struct fanotify_fh are u8.
+	 * Traditionally, filesystem return handle_type < 0xff, but there
+	 * is no enforecement for that in vfs.
+	 */
+	BUILD_BUG_ON(MAX_HANDLE_SZ > 0xff || FILEID_INVALID > 0xff);
+	if (type <= 0 || type >= FILEID_INVALID || fh_len != dwords << 2)
 		goto out_err;
 
 	fh->type = type;