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xen/blkif: Avoid double access to src->nr_segments 

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@eu.citrix.com>
This commit is contained in:
Stefano Stabellini 2015-12-18 15:09:58 +00:00
parent 18f49881cf
commit f9e98e5d7a
1 changed files with 8 additions and 4 deletions
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12
hw/block/xen_blkif.h
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@ -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];
}