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xenbus/xenbus_backend: Disallow pending watch messages 

commit 9996bd4947 upstream.

'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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
5.4-rM2-2.2.x-imx-squashed
SeongJae Park 2020-12-14 10:08:40 +01:00 committed by Greg Kroah-Hartman
parent d3eaea062b
commit 68d139a974
1 changed files with 7 additions and 0 deletions
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7
drivers/xen/xenbus/xenbus_probe_backend.c
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@ -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",