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From: Paolo Bonzini <pbonzini@redhat.com>
To: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: xen-devel@lists.xensource.com, Tim Deegan <Tim.Deegan@citrix.com>
Subject: Re: [PATCH] Xen: fix various checks of unsigned integers < 0
Date: Fri, 29 Oct 2010 23:23:07 +0200	[thread overview]
Message-ID: <4CCB3B3B.3080801@redhat.com> (raw)
In-Reply-To: <2e35ec26-1314-4f30-b172-fbd9b08134c9@default>

On 10/29/2010 05:38 PM, Dan Magenheimer wrote:
> Wow, I wonder how many times this code has executed
> and returned the wrong (incorrectly sign-extended) value?

Probably never---which doesn't make the fix worthless, but is still 
never. :)  The emulator is mostly used for real mode and MMIO, but this 
is long-mode code (which rules out real mode) and the CQO instruction 
doesn't access memory (which rules out MMIO).

To trigger the bug you probably have to cause a race between a thread 
doing MMIO and a thread replacing the MMIO instruction with a CQO.  It 
can be done fairly reliably on KVM; until they were patched, this trick 
allowed to exploit emulator bugs and go from guest-ring3 to guest-ring0.

Paolo

      reply	other threads:[~2010-10-29 21:23 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-10-29 14:02 [PATCH] Xen: fix various checks of unsigned integers < 0 Tim Deegan
2010-10-29 15:38 ` Dan Magenheimer
2010-10-29 21:23   ` Paolo Bonzini [this message]

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