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>

1 files changed, 10 insertions, 5 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index f9099940c272..41fb1a917b17 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -950,6 +950,8 @@ static int xen_blkbk_parse_indirect(struct blkif_request *req,
                 goto unmap;
 
         for (n = 0, i = 0; n < nseg; n++) {
+                uint8_t first_sect, last_sect;
+
                 if ((n % SEGS_PER_INDIRECT_FRAME) == 0) {
                         /* Map indirect 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);
                 }
                 i = n % SEGS_PER_INDIRECT_FRAME;
+
                 pending_req->segments[n]->gref = segments[i].gref;
-                seg[n].nsec = segments[i].last_sect -
-                        segments[i].first_sect + 1;
-                seg[n].offset = (segments[i].first_sect << 9);
-                if ((segments[i].last_sect >= (XEN_PAGE_SIZE >> 9)) ||
-                    (segments[i].last_sect < segments[i].first_sect)) {
+
+                first_sect = READ_ONCE(segments[i].first_sect);
+                last_sect = READ_ONCE(segments[i].last_sect);
+                if (last_sect >= (XEN_PAGE_SIZE >> 9) || last_sect < first_sect) {
                         rc = -EINVAL;
                         goto unmap;
                 }
+
+                seg[n].nsec = last_sect - first_sect + 1;
+                seg[n].offset = first_sect << 9;
                 preq->nr_sects += seg[n].nsec;
         }