linux-kernel - [PATCH 1/2] KVM: x86/mmu: Set @writable to false for non-visible accesses by L2

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Date:   Wed, 15 Apr 2020 14:44:13 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [PATCH 1/2] KVM: x86/mmu: Set @writable to false for non-visible accesses by L2

Explicitly set @writable to false in try_async_pf() if the GFN->PFN
translation is short-circuited due to the requested GFN not being
visible to L2.

Leaving @writable ('map_writable' in the callers) uninitialized is ok
in that it's never actually consumed, but one has to track it all the
way through set_spte() being short-circuited by set_mmio_spte() to
understand that the uninitialized variable is benign, and relying on
@writable being ignored is an unnecessary risk.  Explicitly setting
@writable also aligns try_async_pf() with __gfn_to_pfn_memslot().

Jim Mattson <jmattson@...gle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/mmu/mmu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index c6ea6032c222..6d6cb9416179 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4090,6 +4090,7 @@ static bool try_async_pf(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
 	 */
 	if (is_guest_mode(vcpu) && !kvm_is_visible_gfn(vcpu->kvm, gfn)) {
 		*pfn = KVM_PFN_NOSLOT;
+		*writable = false;
 		return false;
 	}

-- 
2.26.0