Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

[Thomas Gleixner <tglx@linutronix.de>]

Andy Lutomirski <luto@kernel.org> writes:
> On Fri, Mar 6, 2020 at 6:26 PM Andy Lutomirski <luto@kernel.org> wrote:
>> +               /*
>> +                * We do not set KVM_ASYNC_PF_SEND_ALWAYS.  With the current
>> +                * KVM paravirt ABI, the following scenario is possible:
>> +                *
>> +                * #PF: async page fault (KVM_PV_REASON_PAGE_NOT_PRESENT)
>> +                *  NMI before CR2 or KVM_PF_REASON_PAGE_NOT_PRESENT
>> +                *   NMI accesses user memory, e.g. due to perf
>> +                *    #PF: normal page fault
>> +                *     #PF reads CR2 and apf_reason -- apf_reason should be 0
>> +                *
>> +                *  outer #PF reads CR2 and apf_reason -- apf_reason should be
>> +                *  KVM_PV_REASON_PAGE_NOT_PRESENT
>> +                *
>> +                * There is no possible way that both reads of CR2 and
>> +                * apf_reason get the correct values.  Fixing this would
>> +                * require paravirt ABI changes.
>> +                */
>> +
>
> Upon re-reading my own comment, I think the problem is real, but I
> don't think my patch fixes it.  The outer #PF could just as easily
> have come from user mode.  We may actually need the NMI code (and
> perhaps MCE and maybe #DB too) to save, clear, and restore apf_reason.
> If we do this, then maybe CPL0 async PFs are actually okay, but the
> semantics are so poorly defined that I'm not very confident about
> that.

I think even with the current mode this is fixable on the host side when
it keeps track of the state.

The host knows exactly when it injects a async PF and it can store CR2
and reason of that async PF in flight.

On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0
then it knows that the guest has read CR2 and apf_reason. All good
nothing to worry about.

If not it needs to be careful.

As long as the apf_reason of the last async #PF is not cleared by the
guest no new async #PF can be injected. That's already correct because
in that case IF==0 which prevents a nested async #PF.

If MCE, NMI trigger a real pagefault then the #PF injection needs to
clear apf_reason and set the correct CR2. When that #PF returns then the
old CR2 and apf_reason need to be restored.

I tried to figure out whether any of this logic exists in the KVM code,
but I got completely lost in that code. Maybe I try later today again.

Thanks,

	tglx