linux-kernel - Re: [PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit

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Message-ID: <4c5f4f61-a8d1-a707-8100-e095d3b18398@grsecurity.net>
Date:   Thu, 30 Mar 2023 22:30:05 +0200
From:   Mathias Krause <minipli@...ecurity.net>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit

On 30.03.23 22:15, Mathias Krause wrote:
> [...]
> Maybe it's not a stale CR0 value but the page table walker not taking
> the guest's CR0.WP into account? Maybe a missing role update?

Indeed, it is. This does the trick for me:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 31be188aa842..6a9e90725c84 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8372,6 +8372,9 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)

        init_decode_cache(ctxt);
        vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
+       /* Ensure we're doing page table walks with an up2date MMU role */
+       if ((vcpu->arch.cr0 ^ kvm_read_cr0(vcpu)) == X86_CR0_WP)
+               kvm_init_mmu(vcpu);
 }

 void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)

Very heavy weight and misplaced, but a start :)

It should (1) be limited to VMX as that's the only one that would make
CR0.WP a guest owned bit and (2) limited to emulated instructions that
actually do write operations, as read are harmless, obviously.

Mathias