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

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Message-ID: <1d19de68-a308-c4e4-79ee-e37aee226347@grsecurity.net>
Date:   Fri, 31 Mar 2023 16:18:43 +0200
From:   Mathias Krause <minipli@...ecurity.net>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/6] KVM: VMX: Make CR0.WP a guest owned bit

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