# CVE-2026-89961: powerpc/mm: fix wrong addr_pfn tracking in compound vmemmap population

CVE-2026-89961 is fixed in 2 products on 1 platform, first by DEBIAN-CVE-2026-89961, released September 23, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-89961
- Priority: High
- CVSS: 7.8 (Important)
- EPSS: 0.0018
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 16, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-89961

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.12.111-1 | [DEBIAN-CVE-2026-89961](https://patchblox.com/security/data/fix/linux/debian-cve-2026-89961.md) | Sep 23, 2026 |
| Linux | linux Debian 14 | 7.2.6-1 | [DEBIAN-CVE-2026-89961](https://patchblox.com/security/data/fix/linux/debian-cve-2026-89961.md) | Sep 23, 2026 |
| Linux | linux-6.12 Debian 12.15 (bookworm) | 6.12.111-1~deb12u1 | [DEBIAN-CVE-2026-89961](https://patchblox.com/security/data/fix/linux/debian-cve-2026-89961.md) | Sep 23, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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