---
id: "CVE-2026-98918"
kind: "vulnerability"
aliases: ["CVE-2026-98918"]
title: "Linux kernel DAMON on arm64 can write past a page table page"
severity: "unknown"
cvss: null
epss: 0.00198
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["arm64", "linux-kernel", "runtime", "self-hosted"]
not_affected_if: ["damon-disabled", "x86-64-only-fleet"]
published_at: "2026-10-23T13:17:12+00:00"
first_seen_at: "2026-10-23T21:57:33+00:00"
updated_at: "2026-10-25T12:38:50+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98918", "first_seen_at": "2026-10-23T21:57:33+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-98918", "first_seen_at": "2026-10-25T12:38:50+00:00"}]
---

# Linux kernel DAMON on arm64 can write past a page table page

DAMON's access sampling on arm64 passes an unaligned address into a contiguous-PTE helper, so the walk can clear access bits in page table entries beyond the sampled block. The overrun can cause a kernel crash or silent corruption of whatever memory follows the page table page.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Run `uname -r` and compare against the kernel release that contains the upstream fix for CVE-2026-98918 (unknown from the sources)
- Check whether DAMON is in use with `grep -r CONFIG_DAMON /boot/config-$(uname -r)` (unknown whether this is required)

## Fix

- Update the Linux kernel to a release that includes the upstream commit titled 'mm/damon/ops-common: use a page-aligned address in damon_ptep_mkold()' (exact version unknown from the sources)

## Timeline

- 2026-10-23T21:57:33+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98918
- 2026-10-25T12:38:50+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-98918

## From the sources

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

mm/damon/ops-common: use a page-aligned address in damon_ptep_mkold()

__damon_va_prepare_access_check() picks a random byte address within the
region and stores it in r->sampling_addr.  damon_va_mkold() passes it into
a page table walk, which hands it to damon_ptep_mkold() as the address of
the page to sample:

  damon_va_mkold(mm, r->sampling_addr)
    damon_va_walk_page_range(mm, addr, addr + 1)
      damon_mkold_pmd_entry()
        damon_ptep_mkold(pte, vma, addr)
          ptep_test_and_clear_young(vma, addr, pte)
          mmu_notifier_clear_young(mm, addr, addr + PAGE_SIZE)

For arm64, before commit 6f0e1142173a ("arm64: mm: support batch clearing
of the young flag for large folios"), the contpte helper walked exactly
CONT_PTES entries from the aligned-down page table pointer and used @addr
only to pass down to each entry, so an unaligned value was harmless:

        ptep = contpte_align_down(ptep);
        addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
        for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)

Now the range to walk is derived from @addr instead: end = addr + nr *
PAGE_SIZE, rounded up to CONT_PTE_SIZE.  For a sample in the last page of
a contpte block, the sub-page offset puts end just past the block
boundary, so the round-up lands a whole block further and the walk clears
PTE_AF in CONT_PTES entries beyond the sampled block.  For the last block
in a page table page, those entries are past the end of that page, so the
walk writes into the page that follows.

Triggered by the full 7.1/7.2 kernel selftest suite on arm64 (EC2
c/m6g.4xlarge).  The kernel sometimes crashes at or shortly after the
DAMON test.

What the overrun does depends on the page that happens to follow the page
table, so there is no single signature.  If that page is read-only, the
write faults in the sampling path itself:

  Unable to handle kernel write to read-only memory at virtual address ffff0003c5d2d000
    FSC = 0x0f: level 3 permission fault
    CM = 0, WnR = 1, TnD = 0, TagAccess = 0
  CPU: 10 UID: 0 PID: 3487 Comm: kdamond.2
  pc : contpte_test_and_clear_young_ptes+0x70/0xc0
  lr : damon_ptep_mkold+0x1e8/0x1f8
  Call trace:
   contpte_test_and_clear_young_ptes+0x70/0xc0 (P)
   damon_mkold_pmd_entry+0x150/0x170
   walk_pmd_range+0x110/0x2b0
   walk_pud_range+0x10c/0x208
   walk_pgd_range+0x134/0x258
   __walk_page_range+0x98/0x1b0
   walk_page_range_vma_unsafe+0x90/0x148
   walk_page_range_vma+0x28/0x40
   damon_va_walk_page_range+0x114/0x2b8
   damon_va_prepare_access_checks+0xec/0x1a8
   kdamond_fn+0x534/0x770
   kthread+0x128/0x138
   ret_from_fork+0x10/0x20

Otherwise the page is writable, the PTE_AF clearing succeeds silently and
the damage only surfaces later, in whatever happened to own the page, so
the backtrace is unrelated to DAMON and differs between runs.

Pass a page-aligned address to the ptep_test_and_clear_young() call in
damon_ptep_mkold(), which is the only place DAMON can reach
contpte_test_and_clear_young_ptes() from.  Nothing else sees the aligned
address, and r->sampling_addr itself is left as is, so the sampling and
region bookkeeping semantics are unchanged.

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