linux-kernel - [FYI PATCH] KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

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Date:   Mon, 16 Aug 2021 09:57:32 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     seanjc@...gle.com, Maxim Levitsky <mlevitsk@...hat.com>
Subject: [FYI PATCH] KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

From: Maxim Levitsky <mlevitsk@...hat.com>

If L1 disables VMLOAD/VMSAVE intercepts, and doesn't enable
Virtual VMLOAD/VMSAVE (currently not supported for the nested hypervisor),
then VMLOAD/VMSAVE must operate on the L1 physical memory, which is only
possible by making L0 intercept these instructions.

Failure to do so allowed the nested guest to run VMLOAD/VMSAVE unintercepted,
and thus read/write portions of the host physical memory.

Fixes: 89c8a4984fc9 ("KVM: SVM: Enable Virtual VMLOAD VMSAVE feature")

Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/nested.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 28381ca5221c..e5515477c30a 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -158,6 +158,9 @@ void recalc_intercepts(struct vcpu_svm *svm)
 	/* If SMI is not intercepted, ignore guest SMI intercept as well  */
 	if (!intercept_smi)
 		vmcb_clr_intercept(c, INTERCEPT_SMI);
+
+	vmcb_set_intercept(c, INTERCEPT_VMLOAD);
+	vmcb_set_intercept(c, INTERCEPT_VMSAVE);
 }
 
 static void copy_vmcb_control_area(struct vmcb_control_area *dst,
-- 
2.27.0