xen/events: add a proper barrier to 2-level uevent unmasking

commit 4d3fe31bd993ef504350989786858aefdb877daa upstream.

A follow-up patch will require certain write to happen before an event
channel is unmasked.

While the memory barrier is not strictly necessary for all the callers,
the main one will need it. In order to avoid an extra memory barrier
when using fifo event channels, mandate evtchn_unmask() to provide
write ordering.

The 2-level event handling unmask operation is missing an appropriate
barrier, so add it. Fifo event channels are fine in this regard due to
using sync_cmpxchg().

This is part of XSA-332.

Cc: stable@vger.kernel.org
Suggested-by: Julien Grall <julien@xen.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Wei Liu <wl@xen.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/xen/events/events_2l.c b/drivers/xen/events/events_2l.c
index 7dd46312c18023c6e9ca6db16fba107c79a9bb84..04d1b025b29da207ff03736f21ca0ea11ad5cfb0 100644 (file)
--- a/drivers/xen/events/events_2l.c
+++ b/drivers/xen/events/events_2l.c
@@ -90,6 +90,8 @@ static void evtchn_2l_unmask(unsigned port)
 
        BUG_ON(!irqs_disabled());
 
+       smp_wmb();      /* All writes before unmask must be visible. */
+
        if (unlikely((cpu != cpu_from_evtchn(port))))
                do_hypercall = 1;
        else {