KVM: arm64: Fix vma_shift staleness on nested hwpoison path

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: fd276e7 ("KVM: arm64: nv: Handle shadow stage 2 page faults")
Signed-off-by: Fuad Tabba <[email protected]>
Link: https://patch.msgid.link/[email protected]
[maz: simplified vma_shift assignment from the original patch]
Signed-off-by: Marc Zyngier <[email protected]>

Author: Fuad Tabba

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 = __ffs(vma_pagesize);
 	}
 
 	/*