Re: [PATCH v2] KVM: arm64: Reject guest_memfd memslots when the VM has MTE

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

On Mon, Jul 20, 2026 at 04:46:33PM +0100, Marc Zyngier wrote:
> On Mon, 20 Jul 2026 14:09:42 +0100,
> Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> > 
> > The user cannot use MTE on VMAs created by mapping a guest_memfd file,
> > as arch_calc_vm_flag_bits() does not set VM_MTE_ALLOWED.
> > 
> > When creating a guest_memfd backed memslot,
> > kvm_arch_prepare_memory_region() rejects the memslot if MTE is enabled for
> > the VM and if guest_memfd has been mapped in a VMA that intersects the
> > memslot.
> > 
> > However, the documentation for KVM_SET_USER_MEMORY_REGION2 explicitly
> > states that the only condition for userspace_addr is for it to be a legal
> > userspace address, but the mapping is not required to be valid nor
> > populated at memslot creation.
> > 
> > If userspace sets userspace_addr to an address that hasn't been mapped, or
> > if userspace_addr belongs to a VMA that isn't backed by the guest_memfd
> > file, or if the VMA doesn't intersect the memslot, memslot creation is
> > successful and KVM ends up with a VM with MTE and guest_memfd-backed
> > memslots.
> > 
> > The same happens if the order is reversed: when userspace enables MTE, KVM
> > does not check if memslots backed by guest_memfd are already present.
> > 
> > Fix both issues by rejecting guest_memfd-backed memslots when MTE is
> > enabled, and by reject MTE when guest_memfd-backed memslots are already
> > present.
> > 
> > Fixes: 32e200bd6e44 ("KVM: arm64: Enable support for guest_memfd backed memory")
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> 
> This looks reasonable to me, but you probably want to capture some of
> that in the documentation one way or another, because this will leave
> the amateur VMM author puzzled for a bit...

Of course, will do that in v3.

Thanks,
Alex