Re: [PATCH v2] xfs: assure zeroed memory buffers for certain kmem allocations

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

On Wed, Sep 18, 2019 at 10:47:33AM -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.
> 
> Introduce a xfs_buf_flag, XBF_ZERO, to indicate a request for a zeroed
> buffer, and 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>
> ---
> v2: zeroed buffer not required for XBF_READ case. Correct placement
>     and rename the XBF_ZERO flag.
> 
>  fs/xfs/xfs_buf.c       | 9 +++++++--
>  fs/xfs/xfs_buf.h       | 2 ++
>  fs/xfs/xfs_trans_buf.c | 2 ++
>  3 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 120ef99d09e8..0d96efff451e 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -345,6 +345,10 @@ xfs_buf_allocate_memory(
>  	unsigned short		page_count, i;
>  	xfs_off_t		start, end;
>  	int			error;
> +	uint			kmflag_mask = 0;
> +
> +	if ((flags & XBF_ZERO) && !(flags & XBF_READ))

The sole caller of xfs_buf_allocate_memory is xfs_buf_get_map.  If
_get_map is called from the *read_buf* functions, they pass in XBF_READ
to ensure buffer contents are read.  The other _get_map callers seem to
be initializing metadata blocks and do not set XBF_READ.

So I wonder, do you even need XBF_ZERO?  Or could this be reduced to:

	if (!(flags & XBF_READ))
		km_flag_mask |= KM_ZERO;

?

--D

> +		kmflag_mask |= KM_ZERO;
>  
>  	/*
>  	 * for buffers that are contained within a single page, just allocate
> @@ -354,7 +358,8 @@ 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;
> @@ -391,7 +396,7 @@ xfs_buf_allocate_memory(
>  		struct page	*page;
>  		uint		retries = 0;
>  retry:
> -		page = alloc_page(gfp_mask);
> +		page = alloc_page(gfp_mask | kmflag_mask);
>  		if (unlikely(page == NULL)) {
>  			if (flags & XBF_READ_AHEAD) {
>  				bp->b_page_count = i;
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index f6ce17d8d848..dccdd653c9dc 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -33,6 +33,7 @@
>  /* flags used only as arguments to access routines */
>  #define XBF_TRYLOCK	 (1 << 16)/* lock requested, but do not wait */
>  #define XBF_UNMAPPED	 (1 << 17)/* do not map the buffer */
> +#define XBF_ZERO	 (1 << 18)/* zeroed buffer required */
>  
>  /* flags used only internally */
>  #define _XBF_PAGES	 (1 << 20)/* backed by refcounted pages */
> @@ -52,6 +53,7 @@ typedef unsigned int xfs_buf_flags_t;
>  	{ XBF_WRITE_FAIL,	"WRITE_FAIL" }, \
>  	{ XBF_TRYLOCK,		"TRYLOCK" },	/* should never be set */\
>  	{ XBF_UNMAPPED,		"UNMAPPED" },	/* ditto */\
> +	{ XBF_ZERO,		"KMEM_ZERO" }, \
>  	{ _XBF_PAGES,		"PAGES" }, \
>  	{ _XBF_KMEM,		"KMEM" }, \
>  	{ _XBF_DELWRI_Q,	"DELWRI_Q" }
> diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
> index b5b3a78ef31c..087d413c1490 100644
> --- a/fs/xfs/xfs_trans_buf.c
> +++ b/fs/xfs/xfs_trans_buf.c
> @@ -123,6 +123,8 @@ xfs_trans_get_buf_map(
>  	xfs_buf_t		*bp;
>  	struct xfs_buf_log_item	*bip;
>  
> +	flags |= XBF_ZERO;
> +
>  	if (!tp)
>  		return xfs_buf_get_map(target, map, nmaps, flags);
>  
> -- 
> 2.21.0
>