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

[Sean Christopherson <seanjc@google.com>]

On Mon, May 17, 2021, Dave Hansen wrote:
> 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);

This isn't correct, vcpu->arch.pkru holds the guest value, the NMI handler needs
to load the host value.  I was thinking KVM would stash away the current host
value, and the NMI handler would check the saved value against rdpkru().

> 	}
> 	...
> 	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.

Yep.  It would also blow away the guest's value if the guest did WRPKU while it
was running since KVM would never get a chance to read/save the guest's new value.
 
> 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.