From: Jan Beulich <jbeulich@suse.com>
Date: Tue, 15 Dec 2020 13:47:28 +0000 (+0100)
Subject: evtchn/FIFO: add 2nd smp_rmb() to evtchn_fifo_word_from_port()
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=6ea37c69c7d3948d9bb6f217235ae8bd767e8c46;p=xen.git

evtchn/FIFO: add 2nd smp_rmb() to evtchn_fifo_word_from_port()

Besides with add_page_to_event_array() the function also needs to
synchronize with evtchn_fifo_init_control() setting both d->evtchn_fifo
and (subsequently) d->evtchn_port_ops.

This is XSA-359 / CVE-2020-29571.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
master commit: dc8b01affd7f6f36d34c3854f51df0847df3ec0e
master date: 2020-12-15 13:42:51 +0100
---

diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index ab9e496696..454ca40743 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -34,6 +34,13 @@ static inline event_word_t *evtchn_fifo_word_from_port(const struct domain *d,
 {
     unsigned int p, w;
 
+    /*
+     * Callers aren't required to hold d->event_lock, so we need to synchronize
+     * with evtchn_fifo_init_control() setting d->evtchn_port_ops /after/
+     * d->evtchn_fifo.
+     */
+    smp_rmb();
+
     if ( unlikely(port >= d->evtchn_fifo->num_evtchns) )
         return NULL;
 
@@ -590,6 +597,10 @@ int evtchn_fifo_init_control(struct evtchn_init_control *init_control)
         if ( rc < 0 )
             goto error;
 
+        /*
+         * This call, as a side effect, synchronizes with
+         * evtchn_fifo_word_from_port().
+         */
         rc = map_control_block(v, gfn, offset);
         if ( rc < 0 )
             goto error;