xen/events: avoid race with raising an event in unmask_evtchn() (c26377e6) · Commits · 戴 / test

In unmask_evtchn(), when the mask bit is cleared after testing for pending and the event becomes pending between the test and clear, then the upcall will not become pending and the event may be lost or delayed. Avoid this by always clearing the mask bit before checking for pending. If a hypercall is needed, remask the event as EVTCHNOP_unmask will only retrigger pending events if they were masked. This fixes a regression introduced in 3.7 by (xen/events: fix unmask_evtchn for PV on HVM guests) which reordered the clear mask and check pending operations. Changes in v2: - set mask before hypercall. Cc: stable@vger.kernel.org Acked-by:

Stefano Stabellini <stefano.stabellini@eu.citrix.com> Signed-off-by:

David Vrabel <david.vrabel@citrix.com> Signed-off-by:

Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/events.c

+15 −5

Original line number	Diff line number	Diff line
		@@ -403,11 +403,23 @@ static void unmask_evtchn(int port)

		if (unlikely((cpu != cpu_from_evtchn(port))))
		do_hypercall = 1;
		else
		else {
		/*
		* Need to clear the mask before checking pending to
		* avoid a race with an event becoming pending.
		*
		* EVTCHNOP_unmask will only trigger an upcall if the
		* mask bit was set, so if a hypercall is needed
		* remask the event.
		*/
		sync_clear_bit(port, BM(&s->evtchn_mask[0]));
		evtchn_pending = sync_test_bit(port, BM(&s->evtchn_pending[0]));

		if (unlikely(evtchn_pending && xen_hvm_domain()))
		if (unlikely(evtchn_pending && xen_hvm_domain())) {
		sync_set_bit(port, BM(&s->evtchn_mask[0]));
		do_hypercall = 1;
		}
		}

		/* Slow path (hypercall) if this is a non-local port or if this is
		* an hvm domain and an event is pending (hvm domains don't have
		@@ -418,8 +430,6 @@ static void unmask_evtchn(int port)
		} else {
		struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);

		sync_clear_bit(port, BM(&s->evtchn_mask[0]));

		/*
		* The following is basically the equivalent of
		* 'hw_resend_irq'. Just like a real IO-APIC we 'lose

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