ext4: always verify the magic number in xattr blocks

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

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 230ba79715f6a77e1ec939e215342be0990e75dd..0263692979ec41b472088dae2d5a61f6a80cded3 100644 (file)
--- 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;