Re: [PATCH v5] xfs: assure zeroed memory buffers for certain kmem allocations

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Oct 04, 2019 at 02:46:40PM -0500, Bill O'Donnell wrote:
> Guarantee zeroed memory buffers for cases where potential memory
> leak to disk can occur. In these cases, kmem_alloc is used and
> doesn't zero the buffer, opening the possibility of information
> leakage to disk.
> 
> Use existing infrastucture (xfs_buf_allocate_memory) to obtain
> the already zeroed buffer from kernel memory.
> 
> This solution avoids the performance issue that would occur if a
> wholesale change to replace kmem_alloc with kmem_zalloc was done.
> 
> Signed-off-by: Bill O'Donnell <billodo@redhat.com>
> ---
> 
> v5: apply KM_ZERO for kmem_alloc_io calls in xfs_log.c and xfs_log_recover.c
> v4: use __GFP_ZERO as part of gfp_mask (instead of KM_ZERO)
> v3: remove XBF_ZERO flag, and instead use XBF_READ flag only.
> v2: zeroed buffer not required for XBF_READ case. Correct placement
>     and rename the XBF_ZERO flag.
> 
>  fs/xfs/xfs_buf.c         | 12 +++++++++++-
>  fs/xfs/xfs_log.c         |  2 +-
>  fs/xfs/xfs_log_recover.c |  2 +-
>  3 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 120ef99d09e8..5d0a68de5fa6 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -345,6 +345,15 @@ xfs_buf_allocate_memory(
>  	unsigned short		page_count, i;
>  	xfs_off_t		start, end;
>  	int			error;
> +	uint			kmflag_mask = 0;
> +
> +	/*
> +	 * assure zeroed buffer for non-read cases.
> +	 */
> +	if (!(flags & XBF_READ)) {
> +		kmflag_mask |= KM_ZERO;
> +		gfp_mask |= __GFP_ZERO;
> +	}
>  
>  	/*
>  	 * for buffers that are contained within a single page, just allocate
> @@ -354,7 +363,8 @@ xfs_buf_allocate_memory(
>  	size = BBTOB(bp->b_length);
>  	if (size < PAGE_SIZE) {
>  		int align_mask = xfs_buftarg_dma_alignment(bp->b_target);
> -		bp->b_addr = kmem_alloc_io(size, align_mask, KM_NOFS);
> +		bp->b_addr = kmem_alloc_io(size, align_mask,
> +					   KM_NOFS | kmflag_mask);
>  		if (!bp->b_addr) {
>  			/* low memory - use alloc_page loop instead */
>  			goto use_alloc_page;
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index a2beee9f74da..641d07f30a27 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1443,7 +1443,7 @@ xlog_alloc_log(
>  		prev_iclog = iclog;
>  
>  		iclog->ic_data = kmem_alloc_io(log->l_iclog_size, align_mask,
> -						KM_MAYFAIL);
> +						KM_MAYFAIL | KM_ZERO);
>  		if (!iclog->ic_data)
>  			goto out_free_iclog;
>  #ifdef DEBUG
> diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
> index 508319039dce..c1a514ffff55 100644
> --- a/fs/xfs/xfs_log_recover.c
> +++ b/fs/xfs/xfs_log_recover.c
> @@ -127,7 +127,7 @@ xlog_alloc_buffer(
>  	if (nbblks > 1 && log->l_sectBBsize > 1)
>  		nbblks += log->l_sectBBsize;
>  	nbblks = round_up(nbblks, log->l_sectBBsize);
> -	return kmem_alloc_io(BBTOB(nbblks), align_mask, KM_MAYFAIL);
> +	return kmem_alloc_io(BBTOB(nbblks), align_mask, KM_MAYFAIL | KM_ZERO);

Most of the xlog_alloc_buffer callers are reading things from disk
looking for log items to replay.  The only caller that actually writes
anything to disk is xlog_clear_stale_blocks?

I suspect we only need KM_ZERO for that one case, but OTOH I suppose log
recovery should be infrequent, so better safe than sorry...

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

--D

>  }
>  
>  /*
> -- 
> 2.21.0
>