CVE-2026-98918 in Linux
Summary
by MITRE • 10/23/2026
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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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified in the Linux kernel involves a memory corruption issue within the Dynamic Analysis on Memory (DAMON) subsystem, specifically affecting arm64 architectures that utilize contiguous page table entries known as contpte. The root cause lies in how DAMON prepares addresses for access checks and subsequently clears the young flag of pages to monitor memory access patterns. During operation, the function __damon_va_prepare_access_check() selects a random byte address within a monitored region and stores it in r->sampling_addr. This unaligned address is then passed through damon_va_mkold(), which initiates a page table walk via damon_va_walk_page_range(). The walk eventually reaches damon_ptep_mkold(), where the address is intended to be used for clearing the young bit on specific pages using ptep_test_and_clear_young() and mmu_notifier_clear_young().
Prior to commit 6f0e1142173a, which introduced support for batch clearing of the young flag for large folios in arm64 memory management, the contpte helper function aligned down the page table pointer before iterating. In that older implementation, the unaligned address was merely passed through and adjusted internally to align with contiguous page entry boundaries, rendering the initial misalignment harmless. However, the updated logic derives the range of entries to walk directly from the provided address by calculating an end point as addr plus nr times PAGE_SIZE, rounded up to CONT_PTE_SIZE. This change introduces a critical flaw when sampling occurs near the boundary of a contpte block. If the sampled byte is in the last page of such a block, adding one page size and rounding up can push the calculated end address past the current block's boundary into the next contiguous block entirely.
This miscalculation results in an out-of-bounds write operation where the kernel attempts to clear the Accessed Flag (PTE_AF) for entries beyond the intended sampled region. In scenarios involving large folios, this means clearing flags on pages that were not part of the original sampling target. The severity and manifestation of this vulnerability depend heavily on what memory resides in the page immediately following the affected page table structure. If the subsequent page is mapped as read-only, the kernel triggers a fatal exception due to an attempt to write to protected memory. This results in a level 3 permission fault with a clear call trace pointing back to contpte_test_and_clear_young_ptes(), leading to immediate system instability or crash of the kdamond thread responsible for DAMON operations.
Conversely, if the adjacent page is writable and not currently mapped as read-only by other subsystems, the out-of-bounds write may succeed silently without raising an exception immediately. In this case, the corruption manifests later when another process attempts to access or modify that specific memory region, leading to data corruption, unpredictable behavior, or crashes unrelated to DAMON itself. This makes debugging difficult because the backtrace will not point directly to the DAMON code but rather to whatever subsystem was interacting with the corrupted page at the time of failure. The vulnerability is classified under CWE-787 Out-of-bounds Write and aligns with ATT&CK technique T1499 Endpoint Denial of Service, as it can lead to system crashes or instability through memory corruption.
The resolution involves modifying damon_ptep_mkold() to ensure that a page-aligned address is passed to ptep_test_and_clear_young(). This adjustment ensures that the contpte helper operates within the correct boundaries of the sampled region without spilling over into adjacent blocks. By aligning the address before it reaches the low-level memory management functions, the kernel prevents the erroneous calculation of end addresses that exceed block limits. The sampling logic and r->sampling_addr remain unchanged to preserve existing semantics for monitoring accuracy, but the internal handling during flag clearing is corrected. This fix eliminates the risk of writing into unintended page table entries or adjacent physical pages, thereby restoring stability to DAMON operations on arm64 systems with large folio support.