Re: [PATCH] KVM: arm64: Drop mte_allowed check during memslot creation

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Feb 26, 2025 at 03:28:26PM +0530, Aneesh Kumar K.V wrote:
> Marc Zyngier <maz@kernel.org> writes:
> > On Mon, 24 Feb 2025 16:44:06 +0000,
> > Aneesh Kumar K.V <aneesh.kumar@kernel.org> wrote:
> >> >> On Mon, Feb 24, 2025 at 12:24:14PM +0000, Marc Zyngier wrote:
> >> >> > > On Mon, Feb 24, 2025 at 03:09:38PM +0530, Aneesh Kumar K.V (Arm) wrote:
> >> >> > > > This change is needed because, without it, users are not able to use MTE
> >> >> > > > with VFIO passthrough (currently the mapping is either Device or
> >> >> > > > NonCacheable for which tag access check is not applied.), as shown
> >> >> > > > below (kvmtool VMM).
[...]
> >> >> > My other concern is that this gives pretty poor consistency to the
> >> >> > guest, which cannot know what can be tagged and what cannot, and
> >> >> > breaks a guarantee that the guest should be able to rely on.
[...]
> >> What if we trigger a memory fault exit with the TAGACCESS flag, allowing
> >> the VMM to use the GPA to retrieve additional details and print extra
> >> information to aid in analysis? BTW, we will do this on the first fault
> >> in cacheable, non-tagged memory even if there is no tagaccess in that
> >> region. This can be further improved using the NoTagAccess series I
> >> posted earlier, which ensures the memory fault exit occurs only on
> >> actual tag access
> >> 
> >> Something like below?
> >
> > Something like that, only with:
> >
> > - a capability informing userspace of this behaviour
> >
> > - a per-VM (or per-VMA) flag as a buy-in for that behaviour
> 
> If we’re looking for a capability based control, could we tie that up to
> FEAT_MTE_PERM? That’s what I did here:
> 
> https://lore.kernel.org/all/20250110110023.2963795-1-aneesh.kumar@kernel.org
> 
> That patch set also addresses the issue mentioned here. Let me know if
> you think this is a better approach

From the patch linked above:

| @@ -2152,7 +2162,8 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
|  		if (!vma)
|  			break;
| 
| -		if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) {
| +		if (kvm_has_mte(kvm) &&
| +		    !kvm_has_mte_perm(kvm) && !kvm_vma_mte_allowed(vma)) {
|  			ret = -EINVAL;
|  			break;
|  		}

we also have the same ABI change every time FEAT_MTE_PERM is present.
TBH, I'd rather have it from the start as per the patch in this thread,
irrespective of FEAT_MTE_PERM. I'm fine, however, with better exit to
VMM information though.

If we don't want to confuse the VMMs, we could skip the
kvm_vma_mte_allowed() check only for VM_ALLOW_ANY_UNCACHED and
VM_PFNMAP vmas, maybe the latter only with FEAT_MTE_PERM. I don't think
the VMM would get it wrong here since a VFIO mmap() would not support
MTE anyway.

-- 
Catalin