ext4: never move the system.data xattr out of the inode body

When expanding the extra isize space, we must never move the
system.data xattr out of the inode body.  For performance reasons, it
doesn't make any sense, and the inline data implementation assumes
that system.data xattr is never in the external xattr block.

This addresses CVE-2018-10880

https://bugzilla.kernel.org/show_bug.cgi?id=200005

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/xattr.c

@@ -2657,6 +2657,11 @@ static int ext4_xattr_make_inode_space(handle_t *handle, struct inode *inode,
 		last = IFIRST(header);
 		/* Find the entry best suited to be pushed into EA block */
 		for (; !IS_LAST_ENTRY(last); last = EXT4_XATTR_NEXT(last)) {
+			/* never move system.data out of the inode */
+			if ((last->e_name_len == 4) &&
+			    (last->e_name_index == EXT4_XATTR_INDEX_SYSTEM) &&
+			    !memcmp(last->e_name, "data", 4))
+				continue;
 			total_size = EXT4_XATTR_LEN(last->e_name_len);
 			if (!last->e_value_inum)
 				total_size += EXT4_XATTR_SIZE(