FAILED: patch "[PATCH] xenbus/xenbus_backend: Disallow pending watch messages" failed to apply to 4.9-stable tree

FAILED: patch "[PATCH] xenbus/xenbus_backend: Disallow pending watch messages" failed to apply to 4.9-stable tree

[gregkh]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 9996bd494794a2fe393e97e7a982388c6249aa76 Mon Sep 17 00:00:00 2001
From: SeongJae Park <sjpark@amazon.de>
Date: Mon, 14 Dec 2020 10:08:40 +0100
Subject: [PATCH] xenbus/xenbus_backend: Disallow pending watch messages

'xenbus_backend' watches 'state' of devices, which is writable by
guests.  Hence, if guests intensively updates it, dom0 will have lots of
pending events that exhausting memory of dom0.  In other words, guests
can trigger dom0 memory pressure.  This is known as XSA-349.  However,
the watch callback of it, 'frontend_changed()', reads only 'state', so
doesn't need to have the pending events.

To avoid the problem, this commit disallows pending watch messages for
'xenbus_backend' using the 'will_handle()' watch callback.

This is part of XSA-349

Cc: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Michael Kurth <mku@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

diff --git a/drivers/xen/xenbus/xenbus_probe_backend.c b/drivers/xen/xenbus/xenbus_probe_backend.c
index 2ba699897e6d..5abded97e1a7 100644
--- a/drivers/xen/xenbus/xenbus_probe_backend.c
+++ b/drivers/xen/xenbus/xenbus_probe_backend.c
@@ -180,6 +180,12 @@ static int xenbus_probe_backend(struct xen_bus_type *bus, const char *type,
 	return err;
 }
 
+static bool frontend_will_handle(struct xenbus_watch *watch,
+				 const char *path, const char *token)
+{
+	return watch->nr_pending == 0;
+}
+
 static void frontend_changed(struct xenbus_watch *watch,
 			     const char *path, const char *token)
 {
@@ -191,6 +197,7 @@ static struct xen_bus_type xenbus_backend = {
 	.levels = 3,		/* backend/type/<frontend>/<id> */
 	.get_bus_id = backend_bus_id,
 	.probe = xenbus_probe_backend,
+	.otherend_will_handle = frontend_will_handle,
 	.otherend_changed = frontend_changed,
 	.bus = {
 		.name		= "xen-backend",