[PATCH v5 5/7] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

Emanuele Giuseppe Esposito
2021-11-03T14:06:38Z

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>
Reviewed-by: Maxim Levitsky <mlevitsk@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 64fb43234e06..cdddd3258ddf 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -248,13 +248,6 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
static bool _nested_vmcb_check_save(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;

@@ -511,15 +504,10 @@ 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;

@@ -534,8 +522,8 @@ 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);
}
}
@@ -649,7 +637,7 @@ 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), from_vmrun);
if (ret)
return ret;
--
2.27.0