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xen-blkback: read from indirect descriptors only once 

Since indirect descriptors are in memory shared with the frontend, the
frontend could alter the first_sect and last_sect values after they have
been validated but before they are recorded in the request.  This may
result in I/O requests that overflow the foreign page, possibly
overwriting local pages when the I/O request is executed.

When parsing indirect descriptors, only read first_sect and last_sect
once.

This is part of XSA155.

CC: stable@vger.kernel.org
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
This commit is contained in:
Roger Pau Monné 2015-11-03 16:40:43 +00:00 committed by Konrad Rzeszutek Wilk
parent 1f13d75ccb
commit 1877914910
1 changed files with 10 additions and 5 deletions
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15
drivers/block/xen-blkback/blkback.c
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@ -950,6 +950,8 @@ static int xen_blkbk_parse_indirect(struct blkif_request *req,
goto unmap; goto unmap;
for (n = 0, i = 0; n < nseg; n++) { for (n = 0, i = 0; n < nseg; n++) {
uint8_t first_sect, last_sect;
if ((n % SEGS_PER_INDIRECT_FRAME) == 0) { if ((n % SEGS_PER_INDIRECT_FRAME) == 0) {
/* Map indirect segments */ /* Map indirect segments */
if (segments) if (segments)
@ -957,15 +959,18 @@ static int xen_blkbk_parse_indirect(struct blkif_request *req,
segments = kmap_atomic(pages[n/SEGS_PER_INDIRECT_FRAME]->page); segments = kmap_atomic(pages[n/SEGS_PER_INDIRECT_FRAME]->page);
} }
i = n % SEGS_PER_INDIRECT_FRAME; i = n % SEGS_PER_INDIRECT_FRAME;
pending_req->segments[n]->gref = segments[i].gref; pending_req->segments[n]->gref = segments[i].gref;
seg[n].nsec = segments[i].last_sect -
segments[i].first_sect + 1; first_sect = READ_ONCE(segments[i].first_sect);
seg[n].offset = (segments[i].first_sect << 9); last_sect = READ_ONCE(segments[i].last_sect);
if ((segments[i].last_sect >= (XEN_PAGE_SIZE >> 9)) || if (last_sect >= (XEN_PAGE_SIZE >> 9) || last_sect < first_sect) {
(segments[i].last_sect < segments[i].first_sect)) {
rc = -EINVAL; rc = -EINVAL;
goto unmap; goto unmap;
} }
seg[n].nsec = last_sect - first_sect + 1;
seg[n].offset = first_sect << 9;
preq->nr_sects += seg[n].nsec; preq->nr_sects += seg[n].nsec;
} }