Re: [PATCH 0/2] Xen/mem_event: Do not rely on the toolstack being bug-free

[Tamas Lengyel <tamas.lengyel@zentific.com>]

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I've also tested the patch with LibVMI and everything works fine. The
pause/unpause reference count now does take effect, so the previous issue I
reported (a paused domain getting unpaused by mem_event_enable) is fixed by
this patch.

One question I have, what if the toolstack wants to unconditionally (force)
unpause a domain? Right now with this patch if someone runs 'xl pause
domain' a couple times he has no other recourse then to issue 'xl unpause
domain' at least the same number of times, or to restart the entire domain.
Might be user-friendlier if there was an override provided in case a domain
got paused a million times by accident.

Cheers,
Tamas


On Thu, Jul 17, 2014 at 10:26 PM, Razvan Cojocaru <rcojocaru@bitdefender.com
> wrote:

> On 07/17/2014 10:01 PM, Aravindh Puthiyaparambil (aravindp) wrote:
> >> Xen performs insufficient validation of the contents of mem_event
> responses
> >>from the toolstack.  As a result, a buggy toolstack could cause Xen to
> walk off
> >> the end of a domain's vcpu list, and get out of sync with vcpu pause
> reference
> >> counts.
> >>
> >> These two fixes are compile tested only, as I have no way to plausibly
> test the
> >> mem-event functionality itself.
> >
> > One easy way of testing is to use the tools/tests/xen-access test
> program which exercises mem_access and thereby mem_event. It is fairly easy
> to run. Bring up a domain and execute " xen-access <domain_id> write|exec".
> But I understand if you are under time constraints and cannot do it. If you
> Cc me on these patches, I will gladly test them for you.
>
> Indeed, our application is very xen-access-like (except quite a bit more
> involved), and I've tested the original patches with 5 different domains
> 3 times over - but it's a well-behaved citizen of the Xen ecosystem and
> there were no gimmicks involved. No mem_events piled up, and there was
> always just one mem_event handler per domain.
>
> Everything went without a hitch, but I did not try to pause the domain
> while it was running or try to trick the hypervisor in any way.
>
>
> Thanks,
> Razvan Cojocaru
>
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>

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