File 5f6a062c-evtchn-relax-port_is_valid.patch of Package xen

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File 5f6a062c-evtchn-relax-port_is_valid.patch of Package xen (Revision d4df08e2f3fe7a86db20018815932372)

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Subject: evtchn: relax port_is_valid()
From: Jan Beulich jbeulich@suse.com Tue Sep 22 16:11:56 2020 +0200
Date: Tue Sep 22 16:11:56 2020 +0200:
Git: e417504febf4ef5cc3442ebaa00da1a5c7939b22

To avoid ports potentially becoming invalid behind the back of certain
other functions (due to ->max_evtchn shrinking) because of
- a guest invoking evtchn_reset() and from a 2nd vCPU opening new
  channels in parallel (see also XSA-343),
- alloc_unbound_xen_event_channel() produced channels living above the
  2-level range (see also XSA-342),
drop the max_evtchns check from port_is_valid(). For a port for which
the function once returned "true", the returned value may not turn into
"false" later on. The function's result may only depend on bounds which
can only ever grow (which is the case for d->valid_evtchns).

This also eliminates a false sense of safety, utilized by some of the
users (see again XSA-343): Without a suitable lock held, d->max_evtchns
may change at any time, and hence deducing that certain other operations
are safe when port_is_valid() returned true is not legitimate. The
opportunities to abuse this may get widened by the change here
(depending on guest and host configuration), but will be taken care of
by the other XSA.

This is XSA-338.

Fixes: 48974e6ce52e ("evtchn: use a per-domain variable for the max number of event channels")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <jgrall@amazon.com>

diff --git a/xen/include/xen/event.h b/xen/include/xen/event.h
index a7798f6765..ce45298377 100644
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -107,8 +107,6 @@ void notify_via_xen_event_channel(struct domain *ld, int lport);
 
 static inline bool_t port_is_valid(struct domain *d, unsigned int p)
 {
-    if ( p >= d->max_evtchns )
-        return 0;
     return p < read_atomic(&d->valid_evtchns);
 }

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File 5f6a062c-evtchn-relax-port_is_valid.patch of Package xen (Revision d4df08e2f3fe7a86db20018815932372)

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