[xen staging-4.19] gnttab: address fallout from XSA-501

[patchbot@xxxxxxx]

commit 6199bad8717aa072feef799e97f731907ac5d116
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Sep 29 17:06:37 2026 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Oct 13 14:14:39 2026 +0100

    gnttab: address fallout from XSA-501
    
    A v1 -> v2 change isn't the only scenario where "status" may go stale in
    map_grant_ref(). Across a v2 -> v1 -> v2 change the status pages would
    have been freed and re-populated, possibly changing the pages used for the
    array. Check the pointer against the expected version-dependent value.
    
    This is XSA-517 / CVE-2026-79610.
    
    Fixes: 96dfeb41774a ("gnttab: cope with version changes racing other 
operations")
    Assisted-by: Claude
    Reported-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
    (cherry picked from commit e0a0823ee0a97f7e3c43dd7307b008eeb6120cbf)
---
 xen/common/grant_table.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 5805338774..1dc50a69b6 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -1348,13 +1348,19 @@ map_grant_ref(
 
     grant_read_lock(rgt);
 
-    if ( unlikely(evaluate_nospec((rgt->gt_version == 1) !=
-                                  (status == &shah->flags))) )
+    /*
+     * After a v1 -> v2 change behind our backs "ref" may now be out of
+     * bounds.  While after a v2 -> v1 -> v2 change "ref" would still be
+     * suitably bounded, the underlying status page may have moved.
+     */
+    if ( evaluate_nospec(rgt->gt_version == 1
+                         ? status != &shah->flags
+                         : (ref >= nr_grant_entries(rgt) ||
+                            status != &status_entry(rgt, ref))) )
     {
         /*
-         * After a v1 -> v2 change behind our backs "ref" may now be out of
-         * bounds.  Recalculate it, but only for reserved entries.  Others
-         * will have been cleared anyway by the version change.
+         * Recalculate status, but only for reserved entries.  Others will
+         * have been cleared anyway by the version change(s).
          */
         if ( ref < GNTTAB_NR_RESERVED_ENTRIES )
             status = evaluate_nospec(rgt->gt_version == 1)
--
generated by git-patchbot for /home/xen/git/xen.git#staging-4.19