diff mbox

xen/blkif: avoid double access to any shared ring request fields

Message ID 5742C30902000078000ED9FC@prv-mh.provo.novell.com (mailing list archive)
State New, archived
Headers show

Commit Message

Jan Beulich May 23, 2016, 6:44 a.m. UTC
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 <jbeulich@suse.com>
xen/blkif: avoid double access to any shared ring request fields

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 <jbeulich@suse.com>

--- 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;
 }

Comments

Stefano Stabellini May 23, 2016, 8:38 a.m. UTC | #1
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 <jbeulich@suse.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

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;
>  }
>  
> 
> 
> 
>
diff mbox

Patch

--- 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;
 }