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

[Marc Zyngier <maz@kernel.org>]

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...

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.