# CVE-2026-89987: mm/huge_memory: transfer the pmd dirty bit to the folio on zap


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-101499, BELL-CVE-2026-89987, DEBIAN-CVE-2026-89987, GHSA-44gm-xvpq-cmx6, UBUNTU-CVE-2026-89987

## What is CVE-2026-89987?

CVE-2026-89987 is a vulnerability in Linux, published on 16 Sep 2026. 

### Official description

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

mm/huge_memory: transfer the pmd dirty bit to the folio on zap

zap_huge_pmd_folio() propagates the pmd young bit to the folio for the
file case, but not the dirty bit.  The pte path does propagate it, in
zap_present_folio_ptes() and so does the pmd split path, in
__split_huge_pmd_locked().

For most file mappings the omission is harmless, because writing to a
shared file mapping goes through page_mkwrite(), which dirties the folio. 
tmpfs is different: it has no page_mkwrite(), and vma_wants_writenotify()
is false for it, so a *read* fault on a MAP_SHARED tmpfs mapping installs
a writable pmd via do_read_fault().  do_read_fault() does not call
fault_dirty_shared_page(), so subsequent stores through that mapping set
only the hardware dirty bit in the pmd and never call folio_mark_dirty(). 
A shmem folio allocated by a fault is marked uptodate but not dirty (see
the clear: block in shmem_get_folio_gfp()), so PG_dirty is never set at
all.

Unmapping such a folio - munmap(), or exit_mmap() when the process dies -
then loses the only record that it was written, because zap_huge_pmd()
drops the pmd without transferring the dirty bit.  Reclaim afterwards sees
a clean shmem folio: the whole swap-out block in shrink_folio_list() is
inside "if (folio_test_dirty(folio))", so pageout() is skipped and the
folio falls into __remove_mapping().  There, folio_is_file_lru() is false
for a swapbacked folio, so no shadow entry is created and
__filemap_remove_folio(folio, NULL) simply empties the i_pages slot.  The
data is freed without ever being written to swap, and the next fault on
that index returns a freshly zeroed folio.

This is silent data loss for any process that keeps state in a MAP_SHARED
tmpfs segment across an unmap - for example a cache handed from one
process generation to the next through /dev/shm.  It requires the folio to
be PMD-mapped, so it only shows up once shmem THP is enabled (which is
what we did in Meta fleet and started noticing crashes); with THP off the
pte path transfers the dirty bit correctly.  It also only becomes visible
when swap is enabled, because with no swap device shmem folios (which are
on the anon LRU) are not scanned by reclaim at all, so the clean folio is
never dropped.

Reproduced on x86_64 with a tmpfs mounted huge=within_size: read-fault a
2MB-backed region, write a known pattern through the resulting mapping,
munmap, force reclaim of the cgroup, then re-map and read back.  Without
this patch the region reads back as zeros and vmstat shows zswpout 0 - the
data was discarded rather than swapped.  With this patch the region reads
back correctly and the pages are swapped out as expected.  With
huge=never, or when the first touch is a write, the test passes either
way.

## Which versions are affected by CVE-2026-89987?


## How do I fix CVE-2026-89987?

Upgrade to a fixed version.

## Is CVE-2026-89987 being exploited?

There is no known exploitation of CVE-2026-89987 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89987 and fix it if it is affected.

What it is: CVE-2026-89987 is a vulnerability in Linux, published on 16 Sep 2026. 
Exploitation: There is no known exploitation of CVE-2026-89987 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89987/ (Markdown: https://insights.sbomify.com/CVE-2026-89987/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89987
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89987
- OSV: https://osv.dev/vulnerability/CVE-2026-89987

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89987/
Summaries are generated and may contain mistakes; the official description above is the record.