[Xen-devel] [PULL 1/2] xen/blkif: Avoid double access to src->nr_segments

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

src is stored in shared memory and src->nr_segments is dereferenced
twice at the end of the function.  If a compiler decides to compile this
into two separate memory accesses then the size limitation could be
bypassed.

Fix it by removing the double access to src->nr_segments.

This is part of XSA-155.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
---
 hw/block/xen_blkif.h |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/hw/block/xen_blkif.h b/hw/block/xen_blkif.h
index 711b692..c68487cb 100644
--- a/hw/block/xen_blkif.h
+++ b/hw/block/xen_blkif.h
@@ -85,8 +85,10 @@ static inline void blkif_get_x86_32_req(blkif_request_t 
*dst, blkif_x86_32_reque
                d->nr_sectors = s->nr_sectors;
                return;
        }
-       if (n > src->nr_segments)
-               n = src->nr_segments;
+       /* 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++)
                dst->seg[i] = src->seg[i];
 }
@@ -106,8 +108,10 @@ static inline void blkif_get_x86_64_req(blkif_request_t 
*dst, blkif_x86_64_reque
                d->nr_sectors = s->nr_sectors;
                return;
        }
-       if (n > src->nr_segments)
-               n = src->nr_segments;
+       /* 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++)
                dst->seg[i] = src->seg[i];
 }
-- 
1.7.10.4


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