On Nov 26, 2016, at 11:39 PM, Eric Biggers <ebiggers@google.com> wrote:
> 
> 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>

Makes sense.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
> fs/ext4/inode.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 861f848..bc99ebe 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4564,10 +4564,12 @@ 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;
> 		}
> @@ -4685,6 +4687,7 @@ 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 {
> --
> 2.8.0.rc3.226.g39d4020
> 


Cheers, Andreas