linux-kernel - [PATCH v2 4/4] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

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Message-Id: <20210917120329.2013766-5-eesposit@redhat.com>
Date:   Fri, 17 Sep 2021 08:03:29 -0400
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>
Subject: [PATCH v2 4/4] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

Use the already checked svm->nested.save cached fields
(EFER, CR0, CR4, ...) instead of vmcb12's in
nested_vmcb02_prepare_save().
This prevents from creating TOC/TOU races, since the
guest could modify the vmcb12 fields.

This also avoids the need of force-setting EFER_SVME in
nested_vmcb02_prepare_save.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
---
 arch/x86/kvm/svm/nested.c | 24 ++++++------------------
 1 file changed, 6 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 7e4cd134946f..9b2c4895d5d9 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -280,13 +280,6 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
 static bool nested_vmcb_valid_sregs(struct kvm_vcpu *vcpu,
 				    struct vmcb_save_area_cached *save)
 {
-	/*
-	 * FIXME: these should be done after copying the fields,
-	 * to avoid TOC/TOU races.  For these save area checks
-	 * the possible damage is limited since kvm_set_cr0 and
-	 * kvm_set_cr4 handle failure; EFER_SVME is an exception
-	 * so it is force-set later in nested_prepare_vmcb_save.
-	 */
 	if (CC(!(save->efer & EFER_SVME)))
 		return false;
 
@@ -488,15 +481,10 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
 
 	kvm_set_rflags(&svm->vcpu, vmcb12->save.rflags | X86_EFLAGS_FIXED);
 
-	/*
-	 * Force-set EFER_SVME even though it is checked earlier on the
-	 * VMCB12, because the guest can flip the bit between the check
-	 * and now.  Clearing EFER_SVME would call svm_free_nested.
-	 */
-	svm_set_efer(&svm->vcpu, vmcb12->save.efer | EFER_SVME);
+	svm_set_efer(&svm->vcpu, svm->nested.save.efer);
 
-	svm_set_cr0(&svm->vcpu, vmcb12->save.cr0);
-	svm_set_cr4(&svm->vcpu, vmcb12->save.cr4);
+	svm_set_cr0(&svm->vcpu, svm->nested.save.cr0);
+	svm_set_cr4(&svm->vcpu, svm->nested.save.cr4);
 
 	svm->vcpu.arch.cr2 = vmcb12->save.cr2;
 
@@ -511,8 +499,8 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
 
 	/* These bits will be set properly on the first execution when new_vmc12 is true */
 	if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_DR))) {
-		svm->vmcb->save.dr7 = vmcb12->save.dr7 | DR7_FIXED_1;
-		svm->vcpu.arch.dr6  = vmcb12->save.dr6 | DR6_ACTIVE_LOW;
+		svm->vmcb->save.dr7 = svm->nested.save.dr7 | DR7_FIXED_1;
+		svm->vcpu.arch.dr6  = svm->nested.save.dr6 | DR6_ACTIVE_LOW;
 		vmcb_mark_dirty(svm->vmcb, VMCB_DR);
 	}
 }
@@ -621,7 +609,7 @@ int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa,
 	nested_vmcb02_prepare_control(svm);
 	nested_vmcb02_prepare_save(svm, vmcb12);
 
-	ret = nested_svm_load_cr3(&svm->vcpu, vmcb12->save.cr3,
+	ret = nested_svm_load_cr3(&svm->vcpu, svm->nested.save.cr3,
 				  nested_npt_enabled(svm), true);
 	if (ret)
 		return ret;
-- 
2.27.0