From 513f86d73855ce556ea9522b6bfd79f87356dc3a Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Wed, 13 Jun 2018 00:51:28 -0400
Subject: [PATCH] ext4: always verify the magic number in xattr blocks
Git-commit: 513f86d73855ce556ea9522b6bfd79f87356dc3a
Patch-mainline: v4.18-rc4
References: bsc#1099844 cve-2018-10879

If there an inode points to a block which is also some other type of
metadata block (such as a block allocation bitmap), the
buffer_verified flag can be set when it was validated as that other
metadata block type; however, it would make a really terrible external
attribute block.  The reason why we use the verified flag is to avoid
constantly reverifying the block.  However, it doesn't take much
overhead to make sure the magic number of the xattr block is correct,
and this will avoid potential crashes.

This addresses CVE-2018-10879.

https://bugzilla.kernel.org/show_bug.cgi?id=200001

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org
Acked-by: Jan Kara <jack@suse.cz>

---
 fs/ext4/xattr.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 230ba79715f6..0263692979ec 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -230,12 +230,12 @@ __ext4_xattr_check_block(struct inode *inode, struct buffer_head *bh,
 {
 	int error = -EFSCORRUPTED;
 
-	if (buffer_verified(bh))
-		return 0;
-
 	if (BHDR(bh)->h_magic != cpu_to_le32(EXT4_XATTR_MAGIC) ||
 	    BHDR(bh)->h_blocks != cpu_to_le32(1))
 		goto errout;
+	if (buffer_verified(bh))
+		return 0;
+
 	error = -EFSBADCRC;
 	if (!ext4_xattr_block_csum_verify(inode, bh))
 		goto errout;
-- 
2.16.4