Re: [PATCH 08/16] KVM: x86/mmu: WARN and skip MMIO cache on private, reserved page faults

["Huang, Kai" <kai.huang@intel.com>]

On 5/03/2024 4:51 am, Sean Christopherson wrote:
> On Fri, Mar 01, 2024, Kai Huang wrote:
>> On 1/03/2024 12:06 pm, Sean Christopherson wrote:
>>> E.g. in this case, KVM will just skip various fast paths because of the RSVD flag,
>>> and treat the fault like a PRIVATE fault.  Hmm, but page_fault_handle_page_track()
>>> would skip write tracking, which could theoretically cause data corruption, so I
>>> guess arguably it would be safer to bail?
>>>
>>> Anyone else have an opinion?  This type of bug should never escape development,
>>> so I'm a-ok effectively killing the VM.  Unless someone has a good argument for
>>> continuing on, I'll go with Kai's suggestion and squash this:
>>>
>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>> index cedacb1b89c5..d796a162b2da 100644
>>> --- a/arch/x86/kvm/mmu/mmu.c
>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>> @@ -5892,8 +5892,10 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
>>>                   error_code |= PFERR_PRIVATE_ACCESS;
>>>           r = RET_PF_INVALID;
>>> -       if (unlikely((error_code & PFERR_RSVD_MASK) &&
>>> -                    !WARN_ON_ONCE(error_code & PFERR_PRIVATE_ACCESS))) {
>>> +       if (unlikely(error_code & PFERR_RSVD_MASK)) {
>>> +               if (WARN_ON_ONCE(error_code & PFERR_PRIVATE_ACCESS))
>>> +                       return -EFAULT;
>>
>> -EFAULT is part of guest_memfd() memory fault ABI.  I didn't think over this
>> thoroughly but do you want to return -EFAULT here?
> 
> Yes, I/we do.  There are many existing paths that can return -EFAULT from KVM_RUN
> without setting run->exit_reason to KVM_EXIT_MEMORY_FAULT.  Userspace is responsible
> for checking run->exit_reason on -EFAULT (and -EHWPOISON), i.e. must be prepared
> to handle a "bare" -EFAULT, where for all intents and purposes "handle" means
> "terminate the guest".

Right.

> 
> That's actually one of the reasons why KVM_EXIT_MEMORY_FAULT exists, it'd require
> an absurd amount of work and churn in KVM to *safely* return useful information
> on *all* -EFAULTs.  FWIW, I had hopes and dreams of actually doing exactly this,
> but have long since abandoned those dreams.

I am not sure whether we need to do that.  Perhaps it made you feel so 
after we changed to use -EFAULT to carry KVM_EXIT_MEMORY_FAULT. :-)

> 
> In other words, KVM_EXIT_MEMORY_FAULT essentially communicates to userspace that
> (a) userspace can likely fix whatever badness triggered the -EFAULT, and (b) that
> KVM is in a state where fixing the underlying problem and resuming the guest is
> safe, e.g. won't corrupt the guest (because KVM is in a half-baked state).
> 

Sure.  One small issue might be that, in a later code check, we actually 
return KVM_EXIT_MEMORY_FAULT when private fault hits RET_PF_EMULATION -- 
see your patch:

[PATCH 01/16] KVM: x86/mmu: Exit to userspace with -EFAULT if private 
fault hits emulation

So here if we just return -EFAULT w/o reporting KVM_EXIT_MEMORY_FAULT 
when private+reserved is hit, it seems there's a little bit 
inconsistency here.

But you may have concern of corrupting guest here as you mentioned above.