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List:       oss-security
Subject:    [oss-security] Xen Security Advisory 358 v4 (CVE-2020-29570) - FIFO event channels control block rel
From:       Xen.org security team <security () xen ! org>
Date:       2020-12-15 12:20:28
Message-ID: E1kp9Jc-0007Bp-9a () xenbits ! xenproject ! org
[Download RAW message or body]

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Hash: SHA256

            Xen Security Advisory CVE-2020-29570 / XSA-358
                               version 4

          FIFO event channels control block related ordering

UPDATES IN VERSION 4
====================

Public release.

ISSUE DESCRIPTION
=================

Recording of the per-vCPU control block mapping maintained by Xen and
that of pointers into the control block is reversed.  The consumer
assumes, seeing the former initialized, that the latter are also ready
for use.

IMPACT
======

Malicious or buggy guest kernels can mount a Denial of Service (DoS)
attack affecting the entire system.

VULNERABLE SYSTEMS
==================

All Xen versions from 4.4 onwards are vulnerable.  Xen versions 4.3 and
earlier are not vulnerable.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Julien Grall of Amazon.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa358.patch           xen-unstable - 4.10

$ sha256sum xsa358*
c8392659f71ea31574f9f82ab80a37e1359e8b8178d7b060167500bfb134eecc  xsa358.meta
ee719ff8dbf30794ddac1464267cb47c1aac7e39da32d82263f4aebc1a9b509b  xsa358.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.

(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable.  This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)

For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
  http://www.xenproject.org/security-policy.html
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["xsa358.meta" (application/octet-stream)]
["xsa358.patch" (application/octet-stream)]

From: Jan Beulich <jbeulich@suse.com>
Subject: evtchn/FIFO: re-order and synchronize (with) map_control_block()

For evtchn_fifo_set_pending()'s check of the control block having been
set to be effective, ordering of respective reads and writes needs to be
ensured: The control block pointer needs to be recorded strictly after
the setting of all the queue heads, and it needs checking strictly
before any uses of them (this latter aspect was already guaranteed).

This is XSA-358 / CVE-2020-29570.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>
---
v3: Drop read-side barrier again, leveraging guest_test_and_set_bit().
v2: Re-base over queue locking re-work.

--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -249,6 +249,10 @@ static void evtchn_fifo_set_pending(stru
             goto unlock;
         }
 
+        /*
+         * This also acts as the read counterpart of the smp_wmb() in
+         * map_control_block().
+         */
         if ( guest_test_and_set_bit(d, EVTCHN_FIFO_LINKED, word) )
             goto unlock;
 
@@ -474,6 +478,7 @@ static int setup_control_block(struct vc
 static int map_control_block(struct vcpu *v, uint64_t gfn, uint32_t offset)
 {
     void *virt;
+    struct evtchn_fifo_control_block *control_block;
     unsigned int i;
     int rc;
 
@@ -484,10 +489,15 @@ static int map_control_block(struct vcpu
     if ( rc < 0 )
         return rc;
 
-    v->evtchn_fifo->control_block = virt + offset;
+    control_block = virt + offset;
 
     for ( i = 0; i <= EVTCHN_FIFO_PRIORITY_MIN; i++ )
-        v->evtchn_fifo->queue[i].head = &v->evtchn_fifo->control_block->head[i];
+        v->evtchn_fifo->queue[i].head = &control_block->head[i];
+
+    /* All queue heads must have been set before setting the control block. */
+    smp_wmb();
+
+    v->evtchn_fifo->control_block = control_block;
 
     return 0;
 }


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