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KVM: nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

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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>
Message-Id: <20211103140527.752797-6-eesposit@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Emanuele Giuseppe Esposito 2021-11-03 10:05:25 -04:00 • committed by Paolo Bonzini
parent b7a3d8b6f4
commit 355d0473b1
1 changed files with 6 additions and 18 deletions
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24
arch/x86/kvm/svm/nested.c
View file

@ -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;