linux-kernel - [PATCH 5.15 058/196] KVM: x86: nSVM: fix potential NULL derefernce on nested migration

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Date:   Mon, 21 Feb 2022 09:48:10 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.15 058/196] KVM: x86: nSVM: fix potential NULL derefernce on nested migration

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

commit e1779c2714c3023e4629825762bcbc43a3b943df upstream.

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>
Message-Id: <20220207155447.840194-3-mlevitsk@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/kvm/svm/nested.c |   26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1357,18 +1357,6 @@ static int svm_set_nested_state(struct k
 	    !nested_vmcb_valid_sregs(vcpu, save))
 		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
@@ -1394,6 +1382,20 @@ static int svm_set_nested_state(struct k
 
 	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: