[xen staging] gnttab: fix array capacity check in gnttab_get_status_frames()

[patchbot@xxxxxxx]

commit ec820035b875cdbedce5e73f481ce65963ede9ed
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Aug 25 14:19:09 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Aug 25 14:19:09 2021 +0200

    gnttab: fix array capacity check in gnttab_get_status_frames()
    
    The number of grant frames is of no interest here; converting the passed
    in op.nr_frames this way means we allow for 8 times as many GFNs to be
    written as actually fit in the array. We would corrupt xlat areas of
    higher vCPU-s (after having faulted many times while trying to write to
    the guard pages between any two areas) for 32-bit PV guests. For HVM
    guests we'd simply crash as soon as we hit the first guard page, as
    accesses to the xlat area are simply memcpy() there.
    
    This is CVE-2021-28699 / XSA-382.
    
    Fixes: 18b1be5e324b ("gnttab: make resource limits per domain")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/grant_table.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 76a78df405..5190461053 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -3254,12 +3254,11 @@ 
gnttab_get_status_frames(XEN_GUEST_HANDLE_PARAM(gnttab_get_status_frames_t) uop,
         goto unlock;
     }
 
-    if ( unlikely(limit_max < grant_to_status_frames(op.nr_frames)) )
+    if ( unlikely(limit_max < op.nr_frames) )
     {
         gdprintk(XENLOG_WARNING,
-                 "grant_to_status_frames(%u) for d%d is too large (%u,%u)\n",
-                 op.nr_frames, d->domain_id,
-                 grant_to_status_frames(op.nr_frames), limit_max);
+                 "nr_status_frames for %pd is too large (%u,%u)\n",
+                 d, op.nr_frames, limit_max);
         op.status = GNTST_general_error;
         goto unlock;
     }
--
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