linux-kernel - [PATCH RESEND 02/30] KVM: x86: nSVM: fix potential NULL derefernce on nested migration

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Date:   Mon,  7 Feb 2022 17:54:19 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Tony Luck <tony.luck@...el.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Ingo Molnar <mingo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        intel-gvt-dev@...ts.freedesktop.org,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Sean Christopherson <seanjc@...gle.com>,
        David Airlie <airlied@...ux.ie>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jim Mattson <jmattson@...gle.com>, x86@...nel.org,
        Daniel Vetter <daniel@...ll.ch>,
        Borislav Petkov <bp@...en8.de>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>, stable@...r.kernel.org
Subject: [PATCH RESEND 02/30] KVM: x86: nSVM: fix potential NULL derefernce on nested migration

Turns out that due to review feedback and/or rebases
I accidentally moved the call to nested_svm_load_cr3 to be too early,
before the NPT is enabled, which is very wrong to do.

KVM can't even access guest memory at that point as nested NPT
is needed for that, and of course it won't initialize the walk_mmu,
which is main issue the patch was addressing.

Fix this for real.

Fixes: 232f75d3b4b5 ("KVM: nSVM: call nested_svm_load_cr3 on nested state load")
Cc: stable@...r.kernel.org

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/svm/nested.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 1218b5a342fc8..39d280e7e80ef 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1457,18 +1457,6 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 	    !__nested_vmcb_check_save(vcpu, &save_cached))
 		goto out_free;
 
-	/*
-	 * While the nested guest CR3 is already checked and set by
-	 * KVM_SET_SREGS, it was set when nested state was yet loaded,
-	 * thus MMU might not be initialized correctly.
-	 * Set it again to fix this.
-	 */
-
-	ret = nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
-				  nested_npt_enabled(svm), false);
-	if (WARN_ON_ONCE(ret))
-		goto out_free;
-
 
 	/*
 	 * All checks done, we can enter guest mode. Userspace provides
@@ -1494,6 +1482,20 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 
 	svm_switch_vmcb(svm, &svm->nested.vmcb02);
 	nested_vmcb02_prepare_control(svm);
+
+	/*
+	 * While the nested guest CR3 is already checked and set by
+	 * KVM_SET_SREGS, it was set when nested state was yet loaded,
+	 * thus MMU might not be initialized correctly.
+	 * Set it again to fix this.
+	 */
+
+	ret = nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
+				  nested_npt_enabled(svm), false);
+	if (WARN_ON_ONCE(ret))
+		goto out_free;
+
+
 	kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
 	ret = 0;
 out_free:
-- 
2.26.3