From: Jan Beulich <jbeulich@suse.com>
Date: Tue, 11 Oct 2022 12:56:29 +0000 (+0200)
Subject: gnttab: correct locking on transitive grant copy error path
X-Git-Tag: archive/raspbian/4.16.2+90-g0d39a6d1ae-1+rpi1^2~28^2~74
X-Git-Url: https://dgit.raspbian.org/%22http://www.example.com/cgi/%22/%22http:/www.example.com/cgi/%22?a=commitdiff_plain;h=32cb81501c8b858fe9a451650804ec3024a8b364;p=xen.git

gnttab: correct locking on transitive grant copy error path

While the comment next to the lock dropping in preparation of
recursively calling acquire_grant_for_copy() mistakenly talks about the
rd == td case (excluded a few lines further up), the same concerns apply
to the calling of release_grant_for_copy() on a subsequent error path.

This is CVE-2022-33748 / XSA-411.

Fixes: ad48fb963dbf ("gnttab: fix transitive grant handling")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
master commit: 6e3aab858eef614a21a782a3b73acc88e74690ea
master date: 2022-10-11 14:29:30 +0200
---

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 4c742cd8fe..d8ca645b96 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2613,9 +2613,8 @@ acquire_grant_for_copy(
                      trans_domid);
 
         /*
-         * acquire_grant_for_copy() could take the lock on the
-         * remote table (if rd == td), so we have to drop the lock
-         * here and reacquire.
+         * acquire_grant_for_copy() will take the lock on the remote table,
+         * so we have to drop the lock here and reacquire.
          */
         active_entry_release(act);
         grant_read_unlock(rgt);
@@ -2652,11 +2651,25 @@ acquire_grant_for_copy(
                           act->trans_gref != trans_gref ||
                           !act->is_sub_page)) )
         {
+            /*
+             * Like above for acquire_grant_for_copy() we need to drop and then
+             * re-acquire the locks here to prevent lock order inversion issues.
+             * Unlike for acquire_grant_for_copy() we don't need to re-check
+             * anything, as release_grant_for_copy() doesn't depend on the grant
+             * table entry: It only updates internal state and the status flags.
+             */
+            active_entry_release(act);
+            grant_read_unlock(rgt);
+
             release_grant_for_copy(td, trans_gref, readonly);
             rcu_unlock_domain(td);
+
+            grant_read_lock(rgt);
+            act = active_entry_acquire(rgt, gref);
             reduce_status_for_pin(rd, act, status, readonly);
             active_entry_release(act);
             grant_read_unlock(rgt);
+
             put_page(*page);
             *page = NULL;
             return ERESTART;