ext4: lock the the xattr block before calculating its checksum

We must lock the xattr block in order to avoid spurious checksum
failures.

https://bugzilla.kernel.org/show_bug.cgi?id=193661

Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
 fs/ext4/xattr.c | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

On Tue, Feb 28, 2017 at 08:56:50AM -0500, Theodore Ts'o wrote:
> We must lock the xattr block in order to avoid spurious checksum
> failures.
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=193661
> 
> Reported-by: Colin Ian King <colin.king@canonical.com>
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Looks good,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>  fs/ext4/xattr.c | 31 ++++++++++++++-----------------
>  1 file changed, 14 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 67636acf7624..3247057ef5ff 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -131,30 +131,27 @@ static __le32 ext4_xattr_block_csum(struct inode *inode,
>  }
>  
>  static int ext4_xattr_block_csum_verify(struct inode *inode,
> -					sector_t block_nr,
> -					struct ext4_xattr_header *hdr)
> +					struct buffer_head *bh)
>  {
> -	if (ext4_has_metadata_csum(inode->i_sb) &&
> -	    (hdr->h_checksum != ext4_xattr_block_csum(inode, block_nr, hdr)))
> -		return 0;
> -	return 1;
> -}
> -
> -static void ext4_xattr_block_csum_set(struct inode *inode,
> -				      sector_t block_nr,
> -				      struct ext4_xattr_header *hdr)
> -{
> -	if (!ext4_has_metadata_csum(inode->i_sb))
> -		return;
> +	struct ext4_xattr_header *hdr = BHDR(bh);
> +	int ret = 1;
>  
> -	hdr->h_checksum = ext4_xattr_block_csum(inode, block_nr, hdr);
> +	if (ext4_has_metadata_csum(inode->i_sb)) {
> +		lock_buffer(bh);
> +		ret = hdr->h_checksum == ext4_xattr_block_csum(inode,
> +							bh->b_blocknr, hdr);
> +		unlock_buffer(bh);
> +	}
> +	return ret;
>  }
>  
>  static inline int ext4_handle_dirty_xattr_block(handle_t *handle,
>  						struct inode *inode,
>  						struct buffer_head *bh)
>  {
> -	ext4_xattr_block_csum_set(inode, bh->b_blocknr, BHDR(bh));
> +	if (ext4_has_metadata_csum(inode->i_sb))
> +		BHDR(bh)->h_checksum = ext4_xattr_block_csum(inode,
> +						bh->b_blocknr, BHDR(bh));
>  	return ext4_handle_dirty_metadata(handle, inode, bh);
>  }
>  
> @@ -233,7 +230,7 @@ ext4_xattr_check_block(struct inode *inode, struct buffer_head *bh)
>  	if (BHDR(bh)->h_magic != cpu_to_le32(EXT4_XATTR_MAGIC) ||
>  	    BHDR(bh)->h_blocks != cpu_to_le32(1))
>  		return -EFSCORRUPTED;
> -	if (!ext4_xattr_block_csum_verify(inode, bh->b_blocknr, BHDR(bh)))
> +	if (!ext4_xattr_block_csum_verify(inode, bh))
>  		return -EFSBADCRC;
>  	error = ext4_xattr_check_names(BFIRST(bh), bh->b_data + bh->b_size,
>  				       bh->b_data);
> -- 
> 2.11.0.rc0.7.gbe5a750
>