linux-kernel - [PATCH] KVM: nVMX: Prevent vmlaunch with EPTP pointing outside assigned memory area

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Message-Id: <20230628071217.71126-1-ishiir@g.ecc.u-tokyo.ac.jp>
Date:   Wed, 28 Jun 2023 16:12:17 +0900
From:   Reima Ishii <ishiir@...cc.u-tokyo.ac.jp>
To:     unlisted-recipients:; (no To-header on input)
Cc:     ishiir@...cc.u-tokyo.ac.jp, shina@....u-tokyo.ac.jp,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: nVMX: Prevent vmlaunch with EPTP pointing outside assigned memory area

In nested virtualization, if L1 sets an EPTP in VMCS12 that points
beyond the assigned memory area and initiates a vmlaunch to L2, the
existing KVM code handles the vmlaunch, and passes the VMCS
consistency check. However, due to EPTP pointing outside accessible
memory from the vCPU in mmu_check_root(), it attempts to trigger a
triple fault.

As a result, the nested_vmx_triple_fault() and nested_vmx_vmexit() are
called before the actual vmlaunch to L2 occurs. Despite the vmlaunch
not actually being executed in L2, KVM attempts a vmexit to L1,
resulting in a warning in nested_vmx_vmexit() due to the
nested_run_pending flag.

This commit resolves the issue by modifying the nested_vmx_check_eptp()
to return false when EPTP points outside the assigned memory area.
This effectively prevents a vmlaunch with such an EPTP, thus avoiding
the unnecessary warning.

Signed-off-by: Reima Ishii <ishiir@...cc.u-tokyo.ac.jp>
---
 arch/x86/kvm/vmx/nested.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index e35cf0bd0df9..721f98a5dc24 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2727,6 +2727,10 @@ static bool nested_vmx_check_eptp(struct kvm_vcpu *vcpu, u64 new_eptp)
 			return false;
 	}

+	/* Check if EPTP points to an assigned memory area */
+	if (!kvm_vcpu_is_visible_gfn(vcpu, new_eptp >> PAGE_SHIFT))
+		return false;
+
 	return true;
 }

-- 
2.34.1