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

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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>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
This commit is contained in:
Jan Beulich 2016-05-23 00:44:57 -06:00 committed by Stefano Stabellini
parent 55e5c3a2d2
commit 4837a1a516
2 changed files with 8 additions and 6 deletions
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12
hw/block/xen_blkif.h
View file

@ -79,14 +79,14 @@ static inline void blkif_get_x86_32_req(blkif_request_t *dst, blkif_x86_32_reque
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(blkif_request_t *dst, blkif_x86_64_reque
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++)

2
hw/block/xen_disk.c
View file

@ -679,6 +679,8 @@ static int blk_get_request(struct XenBlkDev *blkdev, struct ioreq *ioreq, RING_I
RING_GET_REQUEST(&blkdev->rings.x86_64_part, rc));
break;
}
/* Prevent the compiler from accessing the on-ring fields instead. */
barrier();
return 0;
}