ext4: forbid i_extra_isize not divisible by 4

i_extra_isize not divisible by 4 is problematic for several reasons:

- It causes the in-inode xattr space to be misaligned, but the xattr
  header and entries are not declared __packed to express this
  possibility.  This may cause poor performance or incorrect code
  generation on some platforms.
- When validating the xattr entries we can read past the end of the
  inode if the size available for xattrs is not a multiple of 4.
- It allows the nonsensical i_extra_isize=1, which doesn't even leave
  enough room for i_extra_isize itself.

Therefore, update ext4_iget() to consider i_extra_isize not divisible by
4 to be an error, like the case where i_extra_isize is too large.

This also matches the rule recently added to e2fsck for determining
whether an inode has valid i_extra_isize.

This patch shouldn't have any noticeable effect on
non-corrupted/non-malicious filesystems, since the size of ext4_inode
has always been a multiple of 4.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>

fs/ext4/inode.c

@@ -4572,10 +4572,12 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE) {
 		ei->i_extra_isize = le16_to_cpu(raw_inode->i_extra_isize);
 		if (EXT4_GOOD_OLD_INODE_SIZE + ei->i_extra_isize >
-		    EXT4_INODE_SIZE(inode->i_sb)) {
-			EXT4_ERROR_INODE(inode, "bad extra_isize (%u != %u)",
-				EXT4_GOOD_OLD_INODE_SIZE + ei->i_extra_isize,
-				EXT4_INODE_SIZE(inode->i_sb));
+			EXT4_INODE_SIZE(inode->i_sb) ||
+		    (ei->i_extra_isize & 3)) {
+			EXT4_ERROR_INODE(inode,
+					 "bad extra_isize %u (inode size %u)",
+					 ei->i_extra_isize,
+					 EXT4_INODE_SIZE(inode->i_sb));
 			ret = -EFSCORRUPTED;
 			goto bad_inode;
 		}
@@ -4693,6 +4695,7 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE) {
 		if (ei->i_extra_isize == 0) {
 			/* The extra space is currently unused. Use it. */
+			BUILD_BUG_ON(sizeof(struct ext4_inode) & 3);
 			ei->i_extra_isize = sizeof(struct ext4_inode) -
 					    EXT4_GOOD_OLD_INODE_SIZE;
 		} else {