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 <[email protected]>
---
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
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 <[email protected]>
---
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
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 <[email protected]>
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