linux-kernel - [PATCH V3 1/2] KVM: x86/mmu: Verify shadow walk doesn't terminate early in page faults

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Date:   Mon,  6 Sep 2021 20:25:46 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: [PATCH V3 1/2] KVM: x86/mmu: Verify shadow walk doesn't terminate early in  page faults

From: Sean Christopherson <seanjc@...gle.com>

WARN and bail if the shadow walk for faulting in a SPTE terminates early,
i.e. doesn't reach the expected level because the walk encountered a
terminal SPTE.  The shadow walks for page faults are subtle in that they
install non-leaf SPTEs (zapping leaf SPTEs if necessary!) in the loop
body, and consume the newly created non-leaf SPTE in the loop control,
e.g. __shadow_walk_next().  In other words, the walks guarantee that the
walk will stop if and only if the target level is reached by installing
non-leaf SPTEs to guarantee the walk remains valid.

Opportunistically use fault->goal-level instead of it.level in
FNAME(fetch) to further clarify that KVM always installs the leaf SPTE at
the target level.

Reviewed-by: Lai Jiangshan <jiangshanlai@...il.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/kvm/mmu/mmu.c         | 3 +++
 arch/x86/kvm/mmu/paging_tmpl.h | 7 +++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4853c033e6ce..538be037549d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3006,6 +3006,9 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 			account_huge_nx_page(vcpu->kvm, sp);
 	}
 
+	if (WARN_ON_ONCE(it.level != fault->goal_level))
+		return -EFAULT;
+
 	ret = mmu_set_spte(vcpu, it.sptep, ACC_ALL,
 			   fault->write, fault->goal_level, base_gfn, fault->pfn,
 			   fault->prefault, fault->map_writable);
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 50ade6450ace..4d559d2d4d66 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -749,9 +749,12 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 		}
 	}
 
+	if (WARN_ON_ONCE(it.level != fault->goal_level))
+		return -EFAULT;
+
 	ret = mmu_set_spte(vcpu, it.sptep, gw->pte_access, fault->write,
-			   it.level, base_gfn, fault->pfn, fault->prefault,
-			   fault->map_writable);
+			   fault->goal_level, base_gfn, fault->pfn,
+			   fault->prefault, fault->map_writable);
 	if (ret == RET_PF_SPURIOUS)
 		return ret;
 
-- 
2.19.1.6.gb485710b