File xsa340.patch of Package xen.21908

Overview Repositories Revisions Requests Users Attributes Meta

File xsa340.patch of Package xen.21908

xen/evtchn: Add missing barriers when accessing/allocating an event channel

While the allocation of a bucket is always performed with the per-domain
lock, the bucket may be accessed without the lock taken (for instance, see
evtchn_send()).

Instead such sites relies on port_is_valid() to return a non-zero value
when the port has a struct evtchn associated to it. The function will
mostly check whether the port is less than d->valid_evtchns as all the
buckets/event channels should be allocated up to that point.

Unfortunately a compiler is free to re-order the assignment in
evtchn_allocate_port() so it would be possible to have d->valid_evtchns
updated before the new bucket has finish to allocate.

Additionally on Arm, even if this was compiled "correctly", the
processor can still re-order the memory access.

Add a write memory barrier in the allocation side and a read memory
barrier when the port is valid to prevent any re-ordering issue.

This is XSA-340.

Signed-off-by: Julien Grall <jgrall@amazon.com>

--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -191,6 +191,13 @@ static int get_free_port(struct domain *
         return -ENOMEM;
     bucket_from_port(d, port) = chn;
 
+    /*
+     * d->valid_evtchns is used to check whether the bucket can be
+     * accessed without the per-domain lock. Therefore,
+     * d->valid_evtchns should be seen *after* the new bucket has
+     * been setup.
+     */
+    smp_wmb();
     write_atomic(&d->valid_evtchns, d->valid_evtchns + EVTCHNS_PER_BUCKET);
 
     return port;
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -87,7 +87,17 @@ void arch_evtchn_inject(struct vcpu *v);
 
 static inline bool_t port_is_valid(struct domain *d, unsigned int p)
 {
-    return p < read_atomic(&d->valid_evtchns);
+    if ( p >= read_atomic(&d->valid_evtchns) )
+        return 0;
+
+    /*
+     * The caller will usually access the event channel afterwards and
+     * may be done without taking the per-domain lock. The barrier is
+     * going in pair the smp_wmb() barrier in evtchn_allocate_port().
+     */
+    smp_rmb();
+
+    return 1;
 }
 
 static inline struct evtchn *evtchn_from_port(struct domain *d, unsigned int p)

Places

File xsa340.patch of Package xen.21908

Places