linux-kernel - [PATCH 15/15] KVM: x86/mmu: WARN on NULL pae

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Date:   Tue,  2 Mar 2021 10:45:40 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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, Ben Gardon <bgardon@...gle.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH 15/15] KVM: x86/mmu: WARN on NULL pae_root and bad shadow root level

WARN if KVM is about to derference a NULL pae_root when loading an MMU,
and convert the BUG() on a bad shadow_root_level into a WARN (now that
errors are handled cleanly).  With nested NPT, botching the level and
sending KVM down the wrong path is all too easy, and the on-demand
allocation of pae_root means bugs crash the host.  Obviously, KVM could
unconditionally allocate pae_root, but that's arguably a worse failure
mode as it would potentially corrupt the guest instead of crashing it.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 34eeb39ee0f9..35f89bb1f205 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3247,6 +3247,9 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 				      true);
 		vcpu->arch.mmu->root_hpa = root;
 	} else if (shadow_root_level == PT32E_ROOT_LEVEL) {
+		if (WARN_ON_ONCE(!vcpu->arch.mmu->pae_root))
+			return -EIO;
+
 		for (i = 0; i < 4; ++i) {
 			WARN_ON_ONCE(vcpu->arch.mmu->pae_root[i]);
 
@@ -3256,8 +3259,10 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
 						      shadow_me_mask;
 		}
 		vcpu->arch.mmu->root_hpa = __pa(vcpu->arch.mmu->pae_root);
-	} else
-		BUG();
+	} else {
+		WARN_ONCE(1, "Bad TDP root level = %d\n", shadow_root_level);
+		return -EIO;
+	}
 
 	/* root_pgd is ignored for direct MMUs. */
 	vcpu->arch.mmu->root_pgd = 0;
@@ -3340,6 +3345,8 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
 		vcpu->arch.mmu->lm_root = lm_root;
 
 		lm_root[0] = __pa(vcpu->arch.mmu->pae_root) | pm_mask;
+	} else if (WARN_ON_ONCE(!vcpu->arch.mmu->pae_root)) {
+		return -EIO;
 	}
 
 	for (i = 0; i < 4; ++i) {
-- 
2.30.1.766.gb4fecdf3b7-goog