On Sat, Mar 21, 2020 at 12:22:11PM -0700, Sean Christopherson wrote:
> On Wed, Mar 18, 2020 at 12:37:09PM -0400, Peter Xu wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index e54c6ad628a8..a5123a0aa7d6 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9786,7 +9786,34 @@ void kvm_arch_sync_events(struct kvm *kvm)
> > kvm_free_pit(kvm);
> > }
> >
> > -int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size)
> > +#define ERR_PTR_USR(e) ((void __user *)ERR_PTR(e))
> > +
> > +/**
> > + * __x86_set_memory_region: Setup KVM internal memory slot
> > + *
> > + * @kvm: the kvm pointer to the VM.
> > + * @id: the slot ID to setup.
> > + * @gpa: the GPA to install the slot (unused when @size == 0).
> > + * @size: the size of the slot. Set to zero to uninstall a slot.
> > + *
> > + * This function helps to setup a KVM internal memory slot. Specify
> > + * @size > 0 to install a new slot, while @size == 0 to uninstall a
> > + * slot. The return code can be one of the following:
> > + *
> > + * HVA: on success (uninstall will return a bogus HVA)
> > + * -errno: on error
> > + *
> > + * The caller should always use IS_ERR() to check the return value
> > + * before use. NOTE: KVM internal memory slots are guaranteed and
>
> "are guaranteed" to ...
>
> > + * won't change until the VM is destroyed. This is also true to the
> > + * returned HVA when installing a new memory slot. The HVA can be
> > + * invalidated by either an errornous userspace program or a VM under
> > + * destruction, however as long as we use __copy_{to|from}_user()
> > + * properly upon the HVAs and handle the failure paths always then
> > + * we're safe.
>
> Regarding the HVA, it's a bit confusing saying that it's guaranteed to be
> valid, and then contradicting that in the second clause. Maybe something
> like this to explain the GPA->HVA is guaranteed to be valid, but the
> HVA->HPA is not.
>
> /*
> * before use. Note, KVM internal memory slots are guaranteed to remain valid
> * and unchanged until the VM is destroyed, i.e. the GPA->HVA translation will
> * not change. However, the HVA is a user address, i.e. its accessibility is
> * not guaranteed, and must be accessed via __copy_{to,from}_user().
> */
Sure I can switch to this, though note that I still think the GPA->HVA
is not guaranteed logically because the userspace can unmap any HVA it
wants.. However I agree that shouldn't be important from kvm's
perspective as long as we always emphasize on using legal HVA accessors.
--
Peter Xu
