Re: [PATCH AUTOSEL 4.19 52/68] iomap: sub-block dio needs to zeroout beyond EOF

[Dave Chinner]

On Thu, Nov 29, 2018 at 12:55:43AM -0500, Sasha Levin wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> [ Upstream commit b450672fb66b4a991a5b55ee24209ac7ae7690ce ]
> 
> If we are doing sub-block dio that extends EOF, we need to zero
> the unused tail of the block to initialise the data in it it. If we
> do not zero the tail of the block, then an immediate mmap read of
> the EOF block will expose stale data beyond EOF to userspace. Found
> with fsx running sub-block DIO sizes vs MAPREAD/MAPWRITE operations.
> 
> Fix this by detecting if the end of the DIO write is beyond EOF
> and zeroing the tail if necessary.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  fs/iomap.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap.c b/fs/iomap.c
> index fa46e3ed8f53..82e35265679d 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -1678,7 +1678,14 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		dio->submit.cookie = submit_bio(bio);
>  	} while (nr_pages);
>  
> -	if (need_zeroout) {
> +	/*
> +	 * We need to zeroout the tail of a sub-block write if the extent type
> +	 * requires zeroing or the write extends beyond EOF. If we don't zero
> +	 * the block tail in the latter case, we can expose stale data via mmap
> +	 * reads of the EOF block.
> +	 */
> +	if (need_zeroout ||
> +	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode))) {
>  		/* zero out from the end of the write to the end of the block */
>  		pad = pos & (fs_block_size - 1);
>  		if (pad)

Same again - what's the test plan for these cherry-picked data
corruption fixes?

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx