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>

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index f9099940c2720f95874e6621b4c7e5e1e798fe2c..41fb1a917b172dc9446495d84357060847c2b569 100644 (file)
--- 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;
        }