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SubjectRe: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> 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.

How is the host supposed to know when the #PF returns? Intercepting
IRET sounds like a bad idea and, in any case, is not actually a
reliable indication that #PF returned.

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