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.
