Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave_state

[Dave Hansen]

On 5/17/21 10:39 AM, Sean Christopherson wrote:
> On Mon, May 17, 2021, Paolo Bonzini wrote:
>> On 14/05/21 07:11, Andy Lutomirski wrote:
>>> I don't even want to think about what happens if a perf NMI hits and
>>> accesses host user memory while the guest PKRU is live (on VMX -- I
>>> think this can't happen on SVM).
>> This is indeed a problem, which indeed cannot happen on SVM but is there on
>> VMX.  Note that the function above is not handling all of the xstate, it's
>> handling the *XSAVE state*, that is XCR0, XSS and PKRU.  Thus the window is
>> small, but it's there.
>>
>> Is it solvable at all, without having PKRU fields in the VMCS (and without
>> masking NMIs in the LAPIC which would be too expensive)?  Dave, Sean, what
>> do you think?
> The least awful solution would be to have the NMI handler restore the host's
> PKRU.  The NMI handler would need to save/restore the register, a la CR2, but the
> whole thing could be optimized to run if and only if the NMI lands in the window
> where the guest's PKRU is loaded.

What were you thinking about?  Something like:

	*this_cpu_ptr(&need_nmi_wpkru) = 1
	// Enter Guest
	__write_pkru(vcpu->arch.pkru);
	*this_cpu_ptr(&need_nmi_wpkru) = 0

And then in the NMI handler:

	u32 pkru;

	if (*this_cpu_ptr(&need_nmi_wpkru)) {
		pkru = rdpku();
		__write_pkru(vcpu->arch.pkru);
	}
	...
	copy_*_user_nmi(... whatever ...);
	...
	if (*this_cpu_ptr(&need_nmi_wpkru))
		__write_pkru(pkru);

?

I was thinking we could just blow away PKRU without saving/restoring it
in the NMI handler, but that might clobber PKRU in the window between
need_nmi_wpkru=1 and entering the guest.

But, the save/restore seems doable especially since we can do it in C
and don't have to mess with the NMI stack or anything.