Re: [RFC PATCH v12 07/33] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

[Sean Christopherson]

On Thu, Oct 05, 2023, Anish Moorthy wrote:
> On Tue, Oct 3, 2023 at 4:46 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > The only way a KVM_EXIT_MEMORY_FAULT that actually reaches userspace could be
> > "unreliable" is if something other than a memory_fault exit clobbered the union,
> > but didn't signal its KVM_EXIT_* reason.  And that would be an egregious bug that
> > isn't unique to KVM_EXIT_MEMORY_FAULT, i.e. the same data corruption would affect
> > each and every other KVM_EXIT_* reason.
> 
> Keep in mind the case where an "unreliable" annotation sets up a
> KVM_EXIT_MEMORY_FAULT, KVM_RUN ends up continuing, then something
> unrelated comes up and causes KVM_RUN to EFAULT. Although this at
> least is a case of "outdated" information rather than blatant
> corruption.

Drat, I managed to forget about that.

> IIRC the last time this came up we said that there's minimal harm in
> userspace acting on the outdated info, but it seems like another good
> argument for just restricting the annotations to paths we know are
> reliable. What if the second EFAULT above is fatal (as I understand
> all are today) and sets up subsequent KVM_RUNs to crash and burn
> somehow? Seems like that'd be a safety issue.

For your series, let's omit 

  KVM: Annotate -EFAULTs from kvm_vcpu_read/write_guest_page

and just fill memory_fault for the page fault paths.  That will be easier to
document too since we can simply say that if the exit reason is KVM_EXIT_MEMORY_FAULT,
then run->memory_fault is valid and fresh.

Adding a flag or whatever to mark the data as trustworthy would be the alternative,
but that's effectively adding ABI that says "KVM is buggy, sorry".

My dream of having KVM always return useful information for -EFAULT will have to
wait for another day.