CVE-2026-89961

In the Linux kernel, the following vulnerability has been resolved:

powerpc/mm: fix wrong addr_pfn tracking in compound vmemmap population

vmemmap_populate_compound_pages() uses addr_pfn to determine the PFN
offset within a compound page and to decide whether the current vmemmap
slot should be populated as a head page mapping or should reuse a tail
page mapping.

However, addr_pfn is advanced manually in parallel with addr.  The loop
itself progresses in vmemmap address space, so each PAGE_SIZE step in addr
covers PAGE_SIZE / sizeof(struct page) struct page slots.  Since addr_pfn
is compared against nr_pages in data-PFN units, it should advance by the
same number of PFNs.  The existing manual increments do not match that and
therefore do not reliably track the PFN corresponding to the current addr.

As a result, pfn_offset can be computed from the wrong PFN and the code
can make the head/tail decision for the wrong compound-page position.

Fix this by deriving addr_pfn directly from the current vmemmap address
instead of carrying it as loop state.