Cloudanix VulnDB

CVE-2026-98918

None

CVSS

—

EPSS

0.2%

Published

Oct 23, 2026

CISA due date

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Cloudanix analysis

Schedule a fix SSVC: Track

A Linux kernel memory-monitoring feature can accidentally write past its own data structures on certain ARM-based servers, causing crashes or unpredictable system behavior.

Exploitation
Local Privilege
Attacker needs
Local User
Complexity
Moderate
Impact
Denial Of Service
Component
OS kernel
Class
Memory Corruption
Remediation
Patch
Effort
Low
Patch risk
Low

What it actually does

Unpage-aligned sampling address passed through DAMON page table walk to damon_ptep_mkold() causes contpte_test_and_clear_young_ptes() on arm64 to compute an end address that overruns a CONT_PTE block, writing PTE_AF bits into the following page table page or triggering permission faults.

Why it matters to the business

Kernel instability or crashes on arm64 systems running DAMON workloads, with potential silent corruption whose effects surface later in unrelated code paths; newly public with no known exploitation yet.

How an attack unfolds

Local workload or test triggers kdamond to pick a sampling_addr at the last page of a contpte block; damon_va_walk_page_range hands the unaligned addr to the contpte helpers whose new range calculation extends the walk past the owning page table page, producing an out-of-bounds write whose observable effect depends on the adjacent page contents.

How to fix it

Apply the upstream fix that passes a page-aligned address to ptep_test_and_clear_young() inside damon_ptep_mkold(); rebuild and deploy updated kernel packages for affected arm64 distributions.

How to confirm the fix worked

Execute the full DAMON selftest suite on arm64 hardware; confirm absence of crashes in kdamond or contpte_test_and_clear_young_ptes and verify sampling behavior remains unchanged.

Typical attack path

  1. Run or induce DAMON monitoring on a VMA containing contiguous PTE blocks
  2. Allow kdamond sampling to land on the final page of a contpte region
  3. Trigger the ptep_test_and_clear_young path that now walks from the unaligned address
  4. Observe either immediate fault or later corruption in the adjacent page table or data page

How to detect it

Search kernel logs and crash dumps for call traces containing damon_ptep_mkold, damon_mkold_pmd_entry, contpte_test_and_clear_young_ptes, or the exact fault message "Unable to handle kernel write to read-only memory" originating from kdamond threads.

  • contpte_test_and_clear_young_ptes+0x70/0xc0
  • damon_ptep_mkold+0x1e8/0x1f8
  • kdamond_fn
  • Unable to handle kernel write to read-only memory at virtual address

What your security team should do

  • Prioritize arm64 kernel updates containing the damon_ptep_mkold alignment change
  • Temporarily disable or reduce DAMON sampling granularity on production arm64 fleets until patched
  • Add stack-trace monitoring for damon_mkold_pmd_entry and contpte_test_and_clear_young_ptes symbols

The record

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.

Affected packages

Ecosystem Package Introduced Fixed