linux-kernel - [RFC PATCH 3/3] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri,  3 Sep 2021 12:20:39 +0200
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: [RFC PATCH 3/3] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

Move the checks done by nested_vmcb_valid_sregs and
nested_vmcb_check_controls directly in enter_svm_guest_mode,
and use svm->nested.save cached fields (EFER, CR0, CR4)
instead of vmcb12's.
This prevents from creating TOC/TOU races.

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 | 23 ++++++-----------------
 1 file changed, 6 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 2491c77203c7..487810cfefde 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 *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;
 
@@ -459,7 +452,8 @@ void nested_vmcb02_compute_g_pat(struct vcpu_svm *svm)
 	svm->nested.vmcb02.ptr->save.g_pat = svm->vmcb01.ptr->save.g_pat;
 }
 
-static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12)
+static void nested_vmcb02_prepare_save(struct vcpu_svm *svm,
+				       struct vmcb *vmcb12)
 {
 	bool new_vmcb12 = false;
 
@@ -488,15 +482,10 @@ 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;
 
@@ -671,7 +660,7 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
 	nested_load_control_from_vmcb12(svm, &vmcb12->control);
 	nested_load_save_from_vmcb12(svm, &vmcb12->save);
 
-	if (!nested_vmcb_valid_sregs(vcpu, &vmcb12->save) ||
+	if (!nested_vmcb_valid_sregs(vcpu, &svm->nested.save) ||
 	    !nested_vmcb_check_controls(vcpu, &svm->nested.ctl)) {
 		vmcb12->control.exit_code    = SVM_EXIT_ERR;
 		vmcb12->control.exit_code_hi = 0;
-- 
2.27.0