Re: [Qemu-devel] [PATCH] xen/blkif: avoid double access to any shared ring request fields

[Stefano Stabellini]

On Mon, 23 May 2016, Jan Beulich wrote:
> Commit f9e98e5d7a ("xen/blkif: Avoid double access to
> src->nr_segments") didn't go far enough: src->operation is also being
> used twice. And nothing was done to prevent the compiler from using the
> source side of the copy done by blk_get_request() (granted that's very
> unlikely).
> 
> Move the barrier()s up, and add another one to blk_get_request().
> 
> Note that for completing XSA-155, the barrier() getting added to
> blk_get_request() would suffice, and hence the changes to xen_blkif.h
> are more like just cleanup. And since, as said, the unpatched code
> getting compiled to something vulnerable is very unlikely (and not
> observed in practice), this isn't being viewed as a new security issue.
> 
> Signed-off-by: Jan Beulich <address@hidden>

Reviewed-by: Stefano Stabellini <address@hidden>

Added to my queue


> --- a/hw/block/xen_blkif.h
> +++ b/hw/block/xen_blkif.h
> @@ -79,14 +79,14 @@ static inline void blkif_get_x86_32_req(
>       dst->handle = src->handle;
>       dst->id = src->id;
>       dst->sector_number = src->sector_number;
> -     if (src->operation == BLKIF_OP_DISCARD) {
> +     /* Prevent the compiler from using src->... instead. */
> +     barrier();
> +     if (dst->operation == BLKIF_OP_DISCARD) {
>               struct blkif_request_discard *s = (void *)src;
>               struct blkif_request_discard *d = (void *)dst;
>               d->nr_sectors = s->nr_sectors;
>               return;
>       }
> -     /* prevent the compiler from optimizing the code and using 
> src->nr_segments instead */
> -     barrier();
>       if (n > dst->nr_segments)
>               n = dst->nr_segments;
>       for (i = 0; i < n; i++)
> @@ -102,14 +102,14 @@ static inline void blkif_get_x86_64_req(
>       dst->handle = src->handle;
>       dst->id = src->id;
>       dst->sector_number = src->sector_number;
> -     if (src->operation == BLKIF_OP_DISCARD) {
> +     /* Prevent the compiler from using src->... instead. */
> +     barrier();
> +     if (dst->operation == BLKIF_OP_DISCARD) {
>               struct blkif_request_discard *s = (void *)src;
>               struct blkif_request_discard *d = (void *)dst;
>               d->nr_sectors = s->nr_sectors;
>               return;
>       }
> -     /* prevent the compiler from optimizing the code and using 
> src->nr_segments instead */
> -     barrier();
>       if (n > dst->nr_segments)
>               n = dst->nr_segments;
>       for (i = 0; i < n; i++)
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -679,6 +679,8 @@ static int blk_get_request(struct XenBlk
>                               RING_GET_REQUEST(&blkdev->rings.x86_64_part, 
> rc));
>          break;
>      }
> +    /* Prevent the compiler from accessing the on-ring fields instead. */
> +    barrier();
>      return 0;
>  }
>  
> 
> 
> 
>