Re: [Xen-devel] Altp2m use with PML can deadlock Xen

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Fri, May 10, 2019 at 8:59 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 10/05/2019 15:53, Razvan Cojocaru wrote:
> > On 5/10/19 5:42 PM, Tamas K Lengyel wrote:
> >> On Thu, May 9, 2019 at 10:19 AM Andrew Cooper
> >> <andrew.cooper3@xxxxxxxxxx> wrote:
> >>>
> >>> On 09/05/2019 14:38, Tamas K Lengyel wrote:
> >>>> Hi all,
> >>>> I'm investigating an issue with altp2m that can easily be reproduced
> >>>> and leads to a hypervisor deadlock when PML is available in hardware.
> >>>> I haven't been able to trace down where the actual deadlock occurs.
> >>>>
> >>>> The problem seem to stem from hvm/vmx/vmcs.c:vmx_vcpu_flush_pml_buffer
> >>>> that calls p2m_change_type_one on all gfns that were recorded the PML
> >>>> buffer. The problem occurs when the PML buffer full vmexit happens
> >>>> while the active p2m is an altp2m. Switching  p2m_change_type_one to
> >>>> work with the altp2m instead of the hostp2m however results in EPT
> >>>> misconfiguration crashes.
> >>>>
> >>>> Adding to the issue is that it seem to only occur when the altp2m has
> >>>> remapped GFNs. Since PML records entries based on GFN leads me to
> >>>> question whether it is safe at all to use PML when altp2m is used with
> >>>> GFN remapping. However, AFAICT the GFNs in the PML buffer are not the
> >>>> remapped GFNs and my understanding is that it should be safe as long
> >>>> as the GFNs being tracked by PML are never the remapped GFNs.
> >>>>
> >>>> Booting Xen with ept=pml=0 resolves the issue.
> >>>>
> >>>> If anyone has any insight into what might be happening, please let
> >>>> me know.
> >>>
> >>>
> >>> I could have sworn that George spotted a problem here and fixed it.  I
> >>> shouldn't be surprised if we have more.
> >>>
> >>> The problem that PML introduced (and this is mostly my fault, as I
> >>> suggested the buggy solution) is that the vmexit handler from one vcpu
> >>> pauses others to drain the PML queue into the dirty bitmap.  Overall I
> >>> wasn't happy with the design and I've got some ideas to improve it, but
> >>> within the scope of how altp2m was engineered, I proposed
> >>> domain_pause_except_self().
> >>>
> >>> As it turns out, that is vulnerable to deadlocks when you get two vcpus
> >>> trying to pause each other and waiting for each other to become
> >>> de-scheduled.
> >>
> >> Makes sense.
> >>
> >>>
> >>> I see this has been reused by the altp2m code, but it *should* be safe
> >>> to deadlocks now that it takes the hypercall_deadlock_mutext.
> >>
> >> Is that already in staging or your x86-next branch? I would like to
> >> verify that the problem is still present or not with that change. I
> >> tested with Xen 4.12 release and that definitely still deadlocks.
> >
> > I don't know if Andrew is talking about this patch (probably not, but
> > it looks at least related):
> >
> > http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=24d5282527f4647907b3572820b5335c15cd0356;hp=29d28b29190ba09d53ae7e475108def84e16e363
> >
>
> I was referring to 29d28b2919 which is also in 4.12 as it turns out.
> That said, 24d5282527 might in practice be the cause of the deadlock, so
> I'd first experiment with taking that fix out.
>
> I know for certain that it won't be tested with PML enabled, because the
> use of PML is incompatible with write-protecting guest pagetables.
>

Sounds like it's the safe bet to just have PML be disabled for when
introspection is used. I would say it would be even better if the use
of PML could be controlled on a per-guest base instead of the current
global on/off switch. That way it could be disabled only for the
introspected domains.

I'll do some more experimentation when I get some free time but two
observations that speak against the vCPUs trying to pause each other
being the culprit is that:
- the deadlock doesn't happen with xen-access' altp2m use, it only
happens when there are remapped gfn's in the altp2m views
- I've added a domain_pause/unpause to the PML flusher before it
enters the flush loop but I still got a deadlock

Tamas

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