linux-kernel - Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under kvm_load_host_xsave

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Message-ID: <YKKvychM0nYV8pOJ@google.com>
Date:   Mon, 17 May 2021 18:02:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jon Kohler <jon@...anix.com>, Babu Moger <babu.moger@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Uros Bizjak <ubizjak@...il.com>,
        Petteri Aimonen <jpa@....mail.kapsi.fi>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Benjamin Thiel <b.thiel@...teo.de>,
        Fan Yang <Fan_Yang@...u.edu.cn>,
        Juergen Gross <jgross@...e.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: add hint to skip hidden rdpkru under
 kvm_load_host_xsave_state

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.