[Xen-devel] [PATCH] Fix read-to-use race condition in shadow cod

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Subject:	[Xen-devel] [PATCH] Fix read-to-use race condition in shadow code
From:	George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Date:	Mon, 12 Apr 2010 17:05:12 +0100
Delivery-date:	Mon, 12 Apr 2010 09:11:28 -0700
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If OOS mode is enabled, after last possible resync, read the guest l1e
one last time.  If it's different than the original read, start over
again.

This fixes a race which can result in inconsistent in-sync shadow
tables, leading to corruption:

v1: take page fault, read gl1e from an out-of-sync PT.
v2: modify gl1e, lowering permissions
[v1,v3]: resync l1 which was just read.
v1: propagate change to l1 shadow using stale gl1e

Now we have an in-sync shadow with more permissions than the guest.

The resync can happen either as a result of a 3rd vcpu doing a cr3
update, or under certain conditions by v1 itself.

This should probably be back-ported to 3.4.x and 4.0.x.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

20100412-unstable-fix-gw-to-propagate-race.diff
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[Xen-devel] [PATCH] Fix read-to-use race condition in shadow code