[xen stable-4.12] evtchn/FIFO: re-order and synchronize (with) map_control_block()

[patchbot@xxxxxxx]

commit 51e9505a55f9f706516d959f76386df7a8877595
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Dec 15 14:31:31 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Dec 15 14:31:31 2020 +0100

    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@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
    master commit: c5e63651fdc706954d920a2d98f74f4a21b46a7e
    master date: 2020-12-15 13:46:37 +0100
---
 xen/common/event_fifo.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 2f5e868b7a..742ca31449 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -228,6 +228,10 @@ static void evtchn_fifo_set_pending(struct vcpu *v, struct 
evtchn *evtchn)
             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;
 
@@ -453,6 +457,7 @@ static int setup_control_block(struct vcpu *v)
 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;
 
@@ -463,10 +468,15 @@ static int map_control_block(struct vcpu *v, uint64_t 
gfn, uint32_t offset)
     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;
 }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12