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@xxxxxxxxxx>
> ---
> 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
>
