[PATCH v2 0/4] KVM: nSVM: avoid TOC/TOU race when checking vmcb12

[PATCH v2 0/4] KVM: nSVM: avoid TOC/TOU race when checking vmcb12

[Emanuele Giuseppe Esposito]

Currently there is a TOC/TOU race between the check of vmcb12's
efer, cr0 and cr4 registers and the later save of their values in
svm_set_*, because the guest could modify the values in the meanwhile.

To solve this issue, this serie introuces and uses svm->nested.save
structure in enter_svm_guest_mode to save the current value of efer,
cr0 and cr4 and later use these to set the vcpu->arch.* state.

Patch 1 just refactor the code to simplify the next two patches,
patch 2 introduces svm->nested.save to cache the efer, cr0 and cr4 fields
and in patch 3 and 4 we use it to avoid TOC/TOU races.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

---
RFC:
* use svm->nested.save instead of local variables.
* not dependent anymore from "KVM: nSVM: remove useless kvm_clear_*_queue"
* simplified patches, we just use the struct and not move the check
  nearer to the TOU.

v2:
* svm->nested.save is a separate struct vmcb_save_area_cached,
  and not vmcb_save_area.
* update also vmcb02->cr3 with svm->nested.save.cr3 

Emanuele Giuseppe Esposito (4):
  KVM: nSVM: move nested_vmcb_check_cr3_cr4 logic in
    nested_vmcb_valid_sregs
  nSVM: introduce smv->nested.save to cache save area fields
  nSVM: use vmcb_save_area_cached in nested_vmcb_valid_sregs()
  nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU
    races

 arch/x86/kvm/svm/nested.c | 95 +++++++++++++++++++++------------------
 arch/x86/kvm/svm/svm.c    |  1 +
 arch/x86/kvm/svm/svm.h    | 12 +++++
 3 files changed, 64 insertions(+), 44 deletions(-)

-- 
2.27.0

[PATCH v2 1/4] KVM: nSVM: move nested_vmcb_check_cr3_cr4 logic in nested_vmcb_valid_sregs

[Emanuele Giuseppe Esposito]

Inline nested_vmcb_check_cr3_cr4 as it is not called by anyone else.
Doing so simplifies next patches.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/kvm/svm/nested.c | 35 +++++++++++++----------------------
 1 file changed, 13 insertions(+), 22 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index e5515477c30a..d2fe65e2a7a4 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -260,27 +260,6 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
 	return true;
 }
 
-static bool nested_vmcb_check_cr3_cr4(struct kvm_vcpu *vcpu,
-				      struct vmcb_save_area *save)
-{
-	/*
-	 * These checks are also performed by KVM_SET_SREGS,
-	 * except that EFER.LMA is not checked by SVM against
-	 * CR0.PG && EFER.LME.
-	 */
-	if ((save->efer & EFER_LME) && (save->cr0 & X86_CR0_PG)) {
-		if (CC(!(save->cr4 & X86_CR4_PAE)) ||
-		    CC(!(save->cr0 & X86_CR0_PE)) ||
-		    CC(kvm_vcpu_is_illegal_gpa(vcpu, save->cr3)))
-			return false;
-	}
-
-	if (CC(!kvm_is_valid_cr4(vcpu, save->cr4)))
-		return false;
-
-	return true;
-}
-
 /* Common checks that apply to both L1 and L2 state.  */
 static bool nested_vmcb_valid_sregs(struct kvm_vcpu *vcpu,
 				    struct vmcb_save_area *save)
@@ -302,7 +281,19 @@ static bool nested_vmcb_valid_sregs(struct kvm_vcpu *vcpu,
 	if (CC(!kvm_dr6_valid(save->dr6)) || CC(!kvm_dr7_valid(save->dr7)))
 		return false;
 
-	if (!nested_vmcb_check_cr3_cr4(vcpu, save))
+	/*
+	 * These checks are also performed by KVM_SET_SREGS,
+	 * except that EFER.LMA is not checked by SVM against
+	 * CR0.PG && EFER.LME.
+	 */
+	if ((save->efer & EFER_LME) && (save->cr0 & X86_CR0_PG)) {
+		if (CC(!(save->cr4 & X86_CR4_PAE)) ||
+		    CC(!(save->cr0 & X86_CR0_PE)) ||
+		    CC(kvm_vcpu_is_illegal_gpa(vcpu, save->cr3)))
+			return false;
+	}
+
+	if (CC(!kvm_is_valid_cr4(vcpu, save->cr4)))
 		return false;
 
 	if (CC(!kvm_valid_efer(vcpu, save->efer)))
-- 
2.27.0

[PATCH v2 2/4] nSVM: introduce smv->nested.save to cache save area fields

[Emanuele Giuseppe Esposito]

This is useful in next patch, to avoid having temporary
copies of vmcb12 registers and passing them manually.

Right now, instead of blindly copying everything,
we just copy EFER, CR0, CR3, CR4, DR6 and DR7. If more fields
will need to be added, it will be more obvious to see
that they must be added in struct vmcb_save_area_cached and
in copy_vmcb_save_area().

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 arch/x86/kvm/svm/nested.c | 24 ++++++++++++++++++++++++
 arch/x86/kvm/svm/svm.c    |  1 +
 arch/x86/kvm/svm/svm.h    | 12 ++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index d2fe65e2a7a4..fcbb66915403 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -194,6 +194,22 @@ static void copy_vmcb_control_area(struct vmcb_control_area *dst,
 	dst->pause_filter_thresh  = from->pause_filter_thresh;
 }
 
+static void copy_vmcb_save_area(struct vmcb_save_area_cached *dst,
+				struct vmcb_save_area *from)
+{
+	/*
+	 * Copy only necessary fields, as we need them
+	 * to avoid TOC/TOU races.
+	 */
+	dst->efer = from->efer;
+	dst->cr0 = from->cr0;
+	dst->cr3 = from->cr3;
+	dst->cr4 = from->cr4;
+
+	dst->dr6 = from->dr6;
+	dst->dr7 = from->dr7;
+}
+
 static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
 {
 	/*
@@ -313,6 +329,12 @@ void nested_load_control_from_vmcb12(struct vcpu_svm *svm,
 	svm->nested.ctl.iopm_base_pa  &= ~0x0fffULL;
 }
 
+void nested_load_save_from_vmcb12(struct vcpu_svm *svm,
+				  struct vmcb_save_area *save)
+{
+	copy_vmcb_save_area(&svm->nested.save, save);
+}
+
 /*
  * Synchronize fields that are written by the processor, so that
  * they can be copied back into the vmcb12.
@@ -647,6 +669,7 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
 		return -EINVAL;
 
 	nested_load_control_from_vmcb12(svm, &vmcb12->control);
+	nested_load_save_from_vmcb12(svm, &vmcb12->save);
 
 	if (!nested_vmcb_valid_sregs(vcpu, &vmcb12->save) ||
 	    !nested_vmcb_check_controls(vcpu, &svm->nested.ctl)) {
@@ -1385,6 +1408,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 
 	svm_copy_vmrun_state(&svm->vmcb01.ptr->save, save);
 	nested_load_control_from_vmcb12(svm, ctl);
+	nested_load_save_from_vmcb12(svm, save);
 
 	svm_switch_vmcb(svm, &svm->nested.vmcb02);
 	nested_vmcb02_prepare_control(svm);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 69639f9624f5..169b930322ef 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4386,6 +4386,7 @@ static int svm_leave_smm(struct kvm_vcpu *vcpu, const char *smstate)
 			vmcb12 = map.hva;
 
 			nested_load_control_from_vmcb12(svm, &vmcb12->control);
+			nested_load_save_from_vmcb12(svm, &vmcb12->save);
 
 			ret = enter_svm_guest_mode(vcpu, vmcb12_gpa, vmcb12);
 			kvm_vcpu_unmap(vcpu, &map, true);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index bd0fe94c2920..9982e6136724 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -103,6 +103,15 @@ struct kvm_vmcb_info {
 	uint64_t asid_generation;
 };
 
+struct vmcb_save_area_cached {
+	u64 efer;
+	u64 cr4;
+	u64 cr3;
+	u64 cr0;
+	u64 dr7;
+	u64 dr6;
+};
+
 struct svm_nested_state {
 	struct kvm_vmcb_info vmcb02;
 	u64 hsave_msr;
@@ -119,6 +128,7 @@ struct svm_nested_state {
 
 	/* cache for control fields of the guest */
 	struct vmcb_control_area ctl;
+	struct vmcb_save_area_cached save;
 
 	bool initialized;
 };
@@ -484,6 +494,8 @@ int nested_svm_check_exception(struct vcpu_svm *svm, unsigned nr,
 int nested_svm_exit_special(struct vcpu_svm *svm);
 void nested_load_control_from_vmcb12(struct vcpu_svm *svm,
 				     struct vmcb_control_area *control);
+void nested_load_save_from_vmcb12(struct vcpu_svm *svm,
+				  struct vmcb_save_area *save);
 void nested_sync_control_from_vmcb02(struct vcpu_svm *svm);
 void nested_vmcb02_compute_g_pat(struct vcpu_svm *svm);
 void svm_switch_vmcb(struct vcpu_svm *svm, struct kvm_vmcb_info *target_vmcb);
-- 
2.27.0

Re: [PATCH v2 2/4] nSVM: introduce smv->nested.save to cache save area fields

[Paolo Bonzini]

On 17/09/21 14:03, Emanuele Giuseppe Esposito wrote:
> +static void copy_vmcb_save_area(struct vmcb_save_area_cached *dst,
> +				struct vmcb_save_area *from)
> +{
> +	/*
> +	 * Copy only necessary fields, as we need them

"Copy only fields that are validated" etc.

Paolo

> +	 * to avoid TOC/TOU races.
> +	 */

[PATCH v2 3/4] nSVM: use vmcb_save_area_cached in nested_vmcb_valid_sregs()

[Emanuele Giuseppe Esposito]

Now that struct vmcb_save_area_cached contains the required
vmcb fields values (done in nested_load_save_from_vmcb12()),
check them to see if they are correct in nested_vmcb_valid_sregs().

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 arch/x86/kvm/svm/nested.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index fcbb66915403..7e4cd134946f 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -278,7 +278,7 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
 
 /* Common checks that apply to both L1 and L2 state.  */
 static bool nested_vmcb_valid_sregs(struct kvm_vcpu *vcpu,
-				    struct vmcb_save_area *save)
+				    struct vmcb_save_area_cached *save)
 {
 	/*
 	 * FIXME: these should be done after copying the fields,
@@ -671,7 +671,7 @@ 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;
@@ -1368,11 +1368,12 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 	 * Validate host state saved from before VMRUN (see
 	 * nested_svm_check_permissions).
 	 */
+	nested_load_save_from_vmcb12(svm, save);
 	if (!(save->cr0 & X86_CR0_PG) ||
 	    !(save->cr0 & X86_CR0_PE) ||
 	    (save->rflags & X86_EFLAGS_VM) ||
-	    !nested_vmcb_valid_sregs(vcpu, save))
-		goto out_free;
+	    !nested_vmcb_valid_sregs(vcpu, &svm->nested.save))
+		goto out_free_save;
 
 	/*
 	 * While the nested guest CR3 is already checked and set by
@@ -1384,7 +1385,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 	ret = nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
 				  nested_npt_enabled(svm), false);
 	if (WARN_ON_ONCE(ret))
-		goto out_free;
+		goto out_free_save;
 
 
 	/*
@@ -1408,12 +1409,15 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
 
 	svm_copy_vmrun_state(&svm->vmcb01.ptr->save, save);
 	nested_load_control_from_vmcb12(svm, ctl);
-	nested_load_save_from_vmcb12(svm, save);
 
 	svm_switch_vmcb(svm, &svm->nested.vmcb02);
 	nested_vmcb02_prepare_control(svm);
 	kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
 	ret = 0;
+
+out_free_save:
+	memset(&svm->nested.save, 0, sizeof(struct vmcb_save_area_cached));
+
 out_free:
 	kfree(save);
 	kfree(ctl);
-- 
2.27.0

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

[Emanuele Giuseppe Esposito]

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@redhat.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

Re: [PATCH v2 0/4] KVM: nSVM: avoid TOC/TOU race when checking vmcb12

[Paolo Bonzini]

On 17/09/21 14:03, Emanuele Giuseppe Esposito wrote:
> Currently there is a TOC/TOU race between the check of vmcb12's
> efer, cr0 and cr4 registers and the later save of their values in
> svm_set_*, because the guest could modify the values in the meanwhile.
> 
> To solve this issue, this serie introuces and uses svm->nested.save
> structure in enter_svm_guest_mode to save the current value of efer,
> cr0 and cr4 and later use these to set the vcpu->arch.* state.
> 
> Patch 1 just refactor the code to simplify the next two patches,
> patch 2 introduces svm->nested.save to cache the efer, cr0 and cr4 fields
> and in patch 3 and 4 we use it to avoid TOC/TOU races.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

Most of my remarks from the RFC still apply, so I will wait for v3. 
Thanks, and sorry for the time between send and review.

Paolo