[PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

[Huang, Kai]

On Wed, 2023-11-01 at 10:36 -0700, Sean Christopherson wrote:
> On Wed, Nov 01, 2023, Kai Huang wrote:
> > 
> > > +7.34 KVM_CAP_MEMORY_FAULT_INFO
> > > +------------------------------
> > > +
> > > +:Architectures: x86
> > > +:Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP.
> > > +
> > > +The presence of this capability indicates that KVM_RUN will fill
> > > +kvm_run.memory_fault if KVM cannot resolve a guest page fault VM-Exit, e.g. if
> > > +there is a valid memslot but no backing VMA for the corresponding host virtual
> > > +address.
> > > +
> > > +The information in kvm_run.memory_fault is valid if and only if KVM_RUN returns
> > > +an error with errno=EFAULT or errno=EHWPOISON *and* kvm_run.exit_reason is set
> > > +to KVM_EXIT_MEMORY_FAULT.
> > 
> > IIUC returning -EFAULT or whatever -errno is sort of KVM internal
> > implementation.
> 
> The errno that is returned to userspace is ABI.  In KVM, it's a _very_ poorly
> defined ABI for the vast majority of ioctls(), but it's still technically ABI.
> KVM gets away with being cavalier with errno because the vast majority of errors
> are considered fatal by userespace, i.e. in most cases, userspace simply doesn't
> care about the exact errno.
> 
> A good example is KVM_RUN with -EINTR; if KVM were to return something other than
> -EINTR on a pending signal or vcpu->run->immediate_exit, userspace would fall over.
> 
> > Is it better to relax the validity of kvm_run.memory_fault when
> > KVM_RUN returns any -errno?
> 
> Not unless there's a need to do so, and if there is then we can update the
> documentation accordingly.  If KVM's ABI is that kvm_run.memory_fault is valid
> for any errno, then KVM would need to purge kvm_run.exit_reason super early in
> KVM_RUN, e.g. to prevent an -EINTR return due to immediate_exit from being
> misinterpreted as KVM_EXIT_MEMORY_FAULT.  And purging exit_reason super early is
> subtly tricky because KVM's (again, poorly documented) ABI is that *some* exit
> reasons are preserved across KVM_RUN with vcpu->run->immediate_exit (or with a
> pending signal).
> 
> https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@google.com
> 
> 

Agreed with not to relax to any errno.  However using -EFAULT as part of ABI
definition seems a little bit dangerous, e.g., someone could accidentally or
mistakenly return -EFAULT in KVM_RUN at early time and/or in a completely
different code path, etc.  -EINTR has well defined meaning, but -EFAULT (which
is "Bad address") seems doesn't but I am not sure either. :-)

One example is, for backing VMA with VM_IO | VM_PFNMAP, hva_to_pfn() returns
KVM_PFN_ERR_FAULT when the kernel cannot get a valid PFN (e.g. when SGX vepc
fault handler failed to allocate EPC) and kvm_handle_error_pfn() will just
return -EFAULT.  If kvm_run.exit_reason isn't purged early then is it possible
to have some issue here?