Re: [PATCH v4] xfs: assure zeroed memory buffers for certain kmem allocations

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

On Thu, Sep 19, 2019 at 04:01:38PM -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>
> ---
> 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 +++++++++++-

/me wakes up and wonders, what about the other kmem_alloc_io callers in
xfs?  I think we need to slip a KM_ZERO into the allocation call when we
allocate the log buffers, right?  What about log recovery?

--D

>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> 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;
> -- 
> 2.21.0
>