Re: [PATCH v1 2/2] KVM: arm64: Fix vma_shift staleness on nested hwpoison path

On Thu, 5 Mar 2026 at 16:08, Marc Zyngier [off-list ref] wrote:

Hi Fuad,

On Wed, 04 Mar 2026 16:22:22 +0000,
Fuad Tabba [off-list ref] wrote:

quoted

When user_mem_abort() handles a nested stage-2 fault, it truncates
vma_pagesize to respect the guest's mapping size. However, the local
variable vma_shift is never updated to match this new size.

If the underlying host page turns out to be hardware poisoned,
kvm_send_hwpoison_signal() is called with the original, larger
vma_shift instead of the actual mapping size. This signals incorrect
poison boundaries to userspace and breaks hugepage memory poison
containment for nested VMs.

Update vma_shift to match the truncated vma_pagesize when operating
on behalf of a nested hypervisor.

Fixes: fd276e71d1e7 ("KVM: arm64: nv: Handle shadow stage 2 page faults")
Signed-off-by: Fuad Tabba <redacted>
---
 arch/arm64/kvm/mmu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index e1d6a4f591a9..b08240e0cab1 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c

@@ -1751,6 +1751,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,

              force_pte = (max_map_size == PAGE_SIZE);
              vma_pagesize = min_t(long, vma_pagesize, max_map_size);
+             vma_shift = force_pte ? PAGE_SHIFT : __ffs(vma_pagesize);

If force_pte is set, then we know that max_map_size == PAGE_SIZE. From
there, vma_pagesize == PAGE_SIZE, since nothing can be smaller.

Is there anything preventing us from having:

                vma_shift = __ffs(vma_pagesize);

and be done with it?

Nope, nothing prevents that. Even simpler and better.

Would you like me to respin it?

Cheers,
/fuad

Thanks,

        M.

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