Re: [PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit

Mathias Krause
2023-03-31T14:18:54Z

On 30.03.23 22:55, Mathias Krause wrote:
> On 30.03.23 22:33, Sean Christopherson wrote:
>> I believe this will remedy the issue. If it does, I'll post a proper patch
>> (likely won't be until next week). Compile tested only.
>>
>> ---
>> arch/x86/kvm/mmu.h | 8 +++++++-
>> arch/x86/kvm/mmu/mmu.c | 14 ++++++++++++++
>> 2 files changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
>> index 89f532516a45..4a303aa735dd 100644
>> --- a/arch/x86/kvm/mmu.h
>> +++ b/arch/x86/kvm/mmu.h
>> @@ -113,6 +113,7 @@ void kvm_init_shadow_ept_mmu(struct kvm_vcpu *vcpu, bool execonly,
>> bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu);
>> int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
>> u64 fault_address, char *insn, int insn_len);
>> +void kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu);
>>
>> int kvm_mmu_load(struct kvm_vcpu *vcpu);
>> void kvm_mmu_unload(struct kvm_vcpu *vcpu);
>> @@ -184,8 +185,13 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>> u64 implicit_access = access & PFERR_IMPLICIT_ACCESS;
>> bool not_smap = ((rflags & X86_EFLAGS_AC) | implicit_access) == X86_EFLAGS_AC;
>> int index = (pfec + (not_smap << PFERR_RSVD_BIT)) >> 1;
>> - bool fault = (mmu->permissions[index] >> pte_access) & 1;
>> u32 errcode = PFERR_PRESENT_MASK;
>> + bool fault;
>> +
>> + if (tdp_enabled)
>> + kvm_mmu_refresh_passthrough_bits(vcpu, mmu);
>> +
>> + fault = (mmu->permissions[index] >> pte_access) & 1;
>>
>> WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
>> if (unlikely(mmu->pkru_mask)) {
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 4c874d4ec68f..2a63b5725f36 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -5186,6 +5186,20 @@ static union kvm_cpu_role kvm_calc_cpu_role(struct kvm_vcpu *vcpu,
>> return role;
>> }
>>
>> +void kvm_mmu_refresh_passthrough_bits(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
>> +{
>> + const bool cr0_wp = kvm_is_cr0_bit_set(vcpu, X86_CR0_WP);
>> +
>> + BUILD_BUG_ON((KVM_MMU_CR0_ROLE_BITS & KVM_POSSIBLE_CR0_GUEST_BITS) != X86_CR0_WP);
>> + BUILD_BUG_ON((KVM_MMU_CR4_ROLE_BITS & KVM_POSSIBLE_CR4_GUEST_BITS));
>> +
>> + if (is_cr0_wp(mmu) == cr0_wp)
>> + return;
>> +
>> + mmu->cpu_role.base.cr0_wp = cr0_wp;
>> + reset_guest_paging_metadata(vcpu, mmu);
>> +}
>> +
>> static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
>> {
>> /* tdp_root_level is architecture forced level, use it if nonzero */
>>
>> base-commit: 27d6845d258b67f4eb3debe062b7dacc67e0c393
>
> I tested a backported version of this patch on v6.1 as that's what I was
> testing with and it worked fine. :)
>
> I'll do more thorough tests tomorrow and actually on kvm-x86/next's HEAD.

I extended the KUT CR0.WP tests and could confirm that (a) commit
fb509f76acc8 ("KVM: VMX: Make CR0.WP a guest owned bit") without the
above change triggers errors both ways in the emulator (denies access
while it should be allowed when CR0.WP=0; allows access while it should
trigger a page fault when CR0.WP=1) and (b) the above patch fixes these.

The tests are available here:
https://lore.kernel.org/kvm/20230331135709.132713-1-minipli@grsecurity.net/

Thanks,
Mathias