[Xen-changelog] [qemu-xen stable-4.2] xen/blkif: Avoid double access to src->nr_segments

[patchbot@xxxxxxx]

commit 74fab2ef4c0ba42af477e9e445c9883cc45cf9e6
Author:     Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
AuthorDate: Fri Dec 18 15:44:58 2015 +0000
Commit:     Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
CommitDate: Fri Dec 18 16:00:32 2015 +0000

    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@xxxxxxxxxxxxx>
---
 hw/xen_blkif.h |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/hw/xen_blkif.h b/hw/xen_blkif.h
index c0f4136..24188f4 100644
--- a/hw/xen_blkif.h
+++ b/hw/xen_blkif.h
@@ -79,8 +79,10 @@ 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 (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];
 }
@@ -94,8 +96,10 @@ 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 (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];
 }
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.2

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