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

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

On 1/03/2024 12:06 pm, Sean Christopherson wrote:
> On Fri, Mar 01, 2024, Kai Huang wrote:
>>
>>
>> On 28/02/2024 3:41 pm, Sean Christopherson wrote:
>>> WARN and skip the emulated MMIO fastpath if a private, reserved page fault
>>> is encountered, as private+reserved should be an impossible combination
>>> (KVM should never create an MMIO SPTE for a private access).
>>>
>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>> ---
>>>    arch/x86/kvm/mmu/mmu.c | 3 ++-
>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>> index bd342ebd0809..9206cfa58feb 100644
>>> --- a/arch/x86/kvm/mmu/mmu.c
>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>> @@ -5866,7 +5866,8 @@ 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)) {
>>> +	if (unlikely((error_code & PFERR_RSVD_MASK) &&
>>> +		     !WARN_ON_ONCE(error_code & PFERR_PRIVATE_ACCESS))) {
>>>    		r = handle_mmio_page_fault(vcpu, cr2_or_gpa, direct);
>>>    		if (r == RET_PF_EMULATE)
>>>    			goto emulate;
>>
>> It seems this will make KVM continue to call kvm_mmu_do_page_fault() when
>> such private+reserve error code actually happens (e.g., due to bug), because
>> @r is still RET_PF_INVALID in such case.
> 
> Yep.
> 
>> Is it better to just return error, e.g., -EINVAL, and give up?
> 
> As long as there is no obvious/immediate danger to the host, no obvious way for
> the "bad" behavior to cause data corruption for the guest, and continuing on has
> a plausible chance of working, then KVM should generally try to continue on and
> not terminate the VM.

Agreed.  But I think sometimes it is hard to tell whether there's any 
dangerous things waiting to happen, because that means we have to sanity 
check a lot of code, and when new patches arrive we need to keep that in 
mind too, which could be a nightmare in terms of maintenance.

> 
> 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?