Re: [PATCH] KVM: x86/mmu: Refresh CR0.WP prior to checking for emulated permission faults

[Mathias Krause <minipli@grsecurity.net>]

On 05.04.23 16:36, Sean Christopherson wrote:
> On Wed, Apr 05, 2023, Mathias Krause wrote:
>> On 05.04.23 02:26, Sean Christopherson wrote:
>>> +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));
>>
>> Just curious, this should assert that we don't run into similar issues
>> if we make more bits of CR4 guest owned?
> 
> Yes?  I'm not sure what you're asking.  BUILD_BUG_ON() is a just more flexible
> version of stiatic_assert(); it only requires that the inputs be compile-time
> constants, not purely "static>
> he above throws an error at compile-time if there is new overlap between the
> CR{0,4} MMU role bits and the possible guest-owned bits.  E.g. adding SMEP to the
> possible guest-owned CR4 bits yields:

Yes, I was just asking about the reasoning behind it as it, obviously,
isn't a problem with the current code. But I ran into it while doing
backports, so thanks for adding it :D

> 
> [...]