linux-kernel - Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

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Message-ID: <db574a30f50a2f556dc983f18f78f28c933fdac7.camel@redhat.com>
Date:   Thu, 14 Jan 2021 13:47:28 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Bandan Das <bsd@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Wei Huang <wei.huang2@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        vkuznets@...hat.com, joro@...tes.org, bp@...en8.de,
        tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, dgilbert@...hat.com
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered
 by VM instructions

On Tue, 2021-01-12 at 15:00 -0500, Bandan Das wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> ...
> > > -	if ((emulation_type & EMULTYPE_VMWARE_GP) &&
> > > -	    !is_vmware_backdoor_opcode(ctxt)) {
> > > -		kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> > > -		return 1;
> > > +	if (emulation_type & EMULTYPE_PARAVIRT_GP) {
> > > +		vminstr = is_vm_instr_opcode(ctxt);
> > > +		if (!vminstr && !is_vmware_backdoor_opcode(ctxt)) {
> > > +			kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> > > +			return 1;
> > > +		}
> > > +		if (vminstr)
> > > +			return vminstr;
> > 
> > I'm pretty sure this doesn't correctly handle a VM-instr in L2 that hits a bad
> > L0 GPA and that L1 wants to intercept.  The intercept bitmap isn't checked until
> > x86_emulate_insn(), and the vm*_interception() helpers expect nested VM-Exits to
> > be handled further up the stack.

Actually IMHO this exactly what we want. We want L0 to always intercept
these #GPs, and hide them from the guest.

What we do need to do (and I prepared and attached a patch for that, is that if we run
a guest, we want to inject corresponding vmexit (like SVM_EXIT_VMRUN)
instead of emulating the instruction.
The attached patch does this, and it made my kvm unit test pass,
even if the test was run in a VM (with an unpatched kernel).

This together with setting that X86_FEATURE_SVME_ADDR_CHK bit for
the guest will allow us to hide that errata completely from the guest
which is a very good thing.
(for example for guests that we can't modify)


Best regards,
	Maxim Levitsky



> > 
> So, the condition is that L2 executes a vmload and #GPs on a reserved address, jumps to L0 - L0 doesn't
> check if L1 has asked for the instruction to be intercepted and goes on with emulating
> vmload and returning back to L2 ?



> 
> > >  	}
> > >  
> > >  	/*
> > > -- 
> > > 2.27.0
> > > 


View attachment "patch.diff" of type "text/x-patch" (1315 bytes)