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

Mathias Krause
2023-03-30T20:55:21Z

On 30.03.23 22:33, Sean Christopherson wrote:
> Duh, this is likely a KVM bug. I expect the issue will go away if you revert
>
> fb509f76acc8 ("KVM: VMX: Make CR0.WP a guest owned bit")
>
> KVM doesn't consume CR0.WP for _its_ MMU, but it does consume CR0.WP for the
> guest walker. By passing through CR0.WP, toggling only CR0.WP will not trap
> (obviously) and thus won't run through kvm_post_set_cr0(), thus resulting in stle
> information due to not invoking kvm_init_mmu().

That reasoning sounds familiar ;)

>
> I'm preetty sure I even called that we needed to refresh the permissions, but then
> obviously forgot to actually make that happen.

:(

>
> 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.

Thanks a lot, Sean!

Mathias