CVE-2026-31398

Status: AnalyzedHigh (7.8)—

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

mm/rmap: fix incorrect pte restoration for lazyfree folios

We batch unmap anonymous lazyfree folios by folio_unmap_pte_batch. If the batch has a mix of writable and non-writable bits, we may end up setting the entire batch writable. Fix this by respecting writable bit during batching.

Although on a successful unmap of a lazyfree folio, the soft-dirty bit is lost, preserve it on pte restoration by respecting the bit during batching, to make the fix consistent w.r.t both writable bit and soft-dirty bit.

I was able to write the below reproducer and crash the kernel. Explanation of reproducer (set 64K mTHP to always):

Read full descriptionShow less

Fault in a 64K large folio. Split the VMA at mid-point with MADV_DONTFORK. fork() - parent points to the folio with 8 writable ptes and 8 non-writable ptes. Merge the VMAs with MADV_DOFORK so that folio_unmap_pte_batch() can determine all the 16 ptes as a batch. Do MADV_FREE on the range to mark the folio as lazyfree. Write to the memory to dirty the pte, eventually rmap will dirty the folio. Then trigger reclaim, we will hit the pte restoration path, and the kernel will crash with the trace given below.

The BUG happens at:

The code path is asking for anonymous page to be mapped writable into the pagetable. The BUG_ON() firing implies that such a writable page has been mapped into the pagetables of more than one process, which breaks anonymous memory/CoW semantics.

Technical details traces, logs and code from the original report

	BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);

[   21.134473] kernel BUG at mm/page_table_check.c:118!
[   21.134497] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
[   21.135917] Modules linked in:
[   21.136085] CPU: 1 UID: 0 PID: 1735 Comm: dup-lazyfree Not tainted 7.0.0-rc1-00116-g018018a17770 #1028 PREEMPT
[   21.136858] Hardware name: linux,dummy-virt (DT)
[   21.137019] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[   21.137308] pc : page_table_check_set+0x28c/0x2a8
[   21.137607] lr : page_table_check_set+0x134/0x2a8
[   21.137885] sp : ffff80008a3b3340
[   21.138124] x29: ffff80008a3b3340 x28: fffffdffc3d14400 x27: ffffd1a55e03d000
[   21.138623] x26: 0040000000000040 x25: ffffd1a55f7dd000 x24: 0000000000000001
[   21.139045] x23: 0000000000000001 x22: 0000000000000001 x21: ffffd1a55f217f30
[   21.139629] x20: 0000000000134521 x19: 0000000000134519 x18: 005c43e000040000
[   21.140027] x17: 0001400000000000 x16: 0001700000000000 x15: 000000000000ffff
[   21.140578] x14: 000000000000000c x13: 005c006000000000 x12: 0000000000000020
[   21.140828] x11: 0000000000000000 x10: 005c000000000000 x9 : ffffd1a55c079ee0
[   21.141077] x8 : 0000000000000001 x7 : 005c03e000040000 x6 : 000000004000ffff
[   21.141490] x5 : ffff00017fffce00 x4 : 0000000000000001 x3 : 0000000000000002
[   21.141741] x2 : 0000000000134510 x1 : 0000000000000000 x0 : ffff0000c08228c0
[   21.141991] Call trace:
[   21.142093]  page_table_check_set+0x28c/0x2a8 (P)
[   21.142265]  __page_table_check_ptes_set+0x144/0x1e8
[   21.142441]  __set_ptes_anysz.constprop.0+0x160/0x1a8
[   21.142766]  contpte_set_ptes+0xe8/0x140
[   21.142907]  try_to_unmap_one+0x10c4/0x10d0
[   21.143177]  rmap_walk_anon+0x100/0x250
[   21.143315]  try_to_unmap+0xa0/0xc8
[   21.143441]  shrink_folio_list+0x59c/0x18a8
[   21.143759]  shrink_lruvec+0x664/0xbf0
[   21.144043]  shrink_node+0x218/0x878
[   21.144285]  __node_reclaim.constprop.0+0x98/0x338
[   21.144763]  user_proactive_reclaim+0x2a4/0x340
[   21.145056]  reclaim_store+0x3c/0x60
[   21.145216]  dev_attr_store+0x20/0x40
[   21.145585]  sysfs_kf_write+0x84/0xa8
[   21.145835]  kernfs_fop_write_iter+0x130/0x1c8
[   21.145994]  vfs_write+0x2b8/0x368
[   21.146119]  ksys_write+0x70/0x110
[   21.146240]  __arm64_sys_write+0x24/0x38
[   21.146380]  invoke_syscall+0x50/0x120
[   21.146513]  el0_svc_common.constprop.0+0x48/0xf8
[   21.146679]  do_el0_svc+0x28/0x40
[   21.146798]  el0_svc+0x34/0x110
[   21.146926]  el0t
---truncated---

CVSS

Version: 3.1
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Base score: 7.8

Exploitation probability (EPSS)

Probability of exploitation in the next 30 days: 0.17%
Percentile among all scored CVEs: 6
Score date: 10/5/2026

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

🎯 ATT&CK techniques

How this vulnerability is exploited and what the attacker gains, in MITRE ATT&CK terms.

ExploitationT1068 Exploitation for Privilege Escalationprivilege escalation85 %
Primary impactT1499.004 Application or System Exploitationimpact75 %
Secondary impactT1561.002 Disk Structure Wipeimpact60 %

Vulnerabilidad local de escalada de privilegios en kernel Linux (AV:L, PR:L) que permite acceso no autorizado a memoria de otros procesos mediante manipulación de PTE, causando DoS y corrupción de datos en el sistema.

Inferred by our analysis agent from the official description, CVSS vector and CWE, and checked by a supervisor. May contain errors.

🛡️ ATT&CK mitigations that cover these techniques

Threat Intelligence Program Filter Network Traffic Execution Prevention Application Isolation and Sandboxing Exploit Protection Update Software

Affected technologies (1)

Linux — Linux Kernel

CWEs

CWE-617

References

Raw JSON (NVD)

Show

{
  "id": "CVE-2026-31398",
  "cveTags": [],
  "metrics": {
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 7.8,
          "attackVector": "LOCAL",
          "baseSeverity": "HIGH",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 5.9,
        "exploitabilityScore": 1.8
      }
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  "affected": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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          "vendor": "Linux",
          "product": "Linux",
          "versions": [
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              "status": "affected",
              "version": "354dffd29575cdf13154e8fb787322354aa9efc4",
              "lessThan": "99888a4f340ca8e839a0524556bd4db76d63f4e0",
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              "lessThan": "29f40594a28114b9a9bc87f6cf7bbee9609628f2",
              "versionType": "git"
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          "programFiles": [
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          "defaultStatus": "unaffected"
        },
        {
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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              "version": "0",
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              "versionType": "semver",
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  "published": "2026-04-03T16:16:38.240",
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      "tags": [
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
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  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "type": "Primary",
      "source": "nvd@nist.gov",
      "description": [
        {
          "lang": "en",
          "value": "CWE-617"
        }
      ]
    }
  ],
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm/rmap: fix incorrect pte restoration for lazyfree folios\n\nWe batch unmap anonymous lazyfree folios by folio_unmap_pte_batch.  If the\nbatch has a mix of writable and non-writable bits, we may end up setting\nthe entire batch writable.  Fix this by respecting writable bit during\nbatching.\n\nAlthough on a successful unmap of a lazyfree folio, the soft-dirty bit is\nlost, preserve it on pte restoration by respecting the bit during\nbatching, to make the fix consistent w.r.t both writable bit and\nsoft-dirty bit.\n\nI was able to write the below reproducer and crash the kernel. \nExplanation of reproducer (set 64K mTHP to always):\n\nFault in a 64K large folio.  Split the VMA at mid-point with\nMADV_DONTFORK.  fork() - parent points to the folio with 8 writable ptes\nand 8 non-writable ptes.  Merge the VMAs with MADV_DOFORK so that\nfolio_unmap_pte_batch() can determine all the 16 ptes as a batch.  Do\nMADV_FREE on the range to mark the folio as lazyfree.  Write to the memory\nto dirty the pte, eventually rmap will dirty the folio.  Then trigger\nreclaim, we will hit the pte restoration path, and the kernel will crash\nwith the trace given below.\n\nThe BUG happens at:\n\n\tBUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);\n\nThe code path is asking for anonymous page to be mapped writable into the\npagetable.  The BUG_ON() firing implies that such a writable page has been\nmapped into the pagetables of more than one process, which breaks\nanonymous memory/CoW semantics.\n\n[   21.134473] kernel BUG at mm/page_table_check.c:118!\n[   21.134497] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP\n[   21.135917] Modules linked in:\n[   21.136085] CPU: 1 UID: 0 PID: 1735 Comm: dup-lazyfree Not tainted 7.0.0-rc1-00116-g018018a17770 #1028 PREEMPT\n[   21.136858] Hardware name: linux,dummy-virt (DT)\n[   21.137019] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)\n[   21.137308] pc : page_table_check_set+0x28c/0x2a8\n[   21.137607] lr : page_table_check_set+0x134/0x2a8\n[   21.137885] sp : ffff80008a3b3340\n[   21.138124] x29: ffff80008a3b3340 x28: fffffdffc3d14400 x27: ffffd1a55e03d000\n[   21.138623] x26: 0040000000000040 x25: ffffd1a55f7dd000 x24: 0000000000000001\n[   21.139045] x23: 0000000000000001 x22: 0000000000000001 x21: ffffd1a55f217f30\n[   21.139629] x20: 0000000000134521 x19: 0000000000134519 x18: 005c43e000040000\n[   21.140027] x17: 0001400000000000 x16: 0001700000000000 x15: 000000000000ffff\n[   21.140578] x14: 000000000000000c x13: 005c006000000000 x12: 0000000000000020\n[   21.140828] x11: 0000000000000000 x10: 005c000000000000 x9 : ffffd1a55c079ee0\n[   21.141077] x8 : 0000000000000001 x7 : 005c03e000040000 x6 : 000000004000ffff\n[   21.141490] x5 : ffff00017fffce00 x4 : 0000000000000001 x3 : 0000000000000002\n[   21.141741] x2 : 0000000000134510 x1 : 0000000000000000 x0 : ffff0000c08228c0\n[   21.141991] Call trace:\n[   21.142093]  page_table_check_set+0x28c/0x2a8 (P)\n[   21.142265]  __page_table_check_ptes_set+0x144/0x1e8\n[   21.142441]  __set_ptes_anysz.constprop.0+0x160/0x1a8\n[   21.142766]  contpte_set_ptes+0xe8/0x140\n[   21.142907]  try_to_unmap_one+0x10c4/0x10d0\n[   21.143177]  rmap_walk_anon+0x100/0x250\n[   21.143315]  try_to_unmap+0xa0/0xc8\n[   21.143441]  shrink_folio_list+0x59c/0x18a8\n[   21.143759]  shrink_lruvec+0x664/0xbf0\n[   21.144043]  shrink_node+0x218/0x878\n[   21.144285]  __node_reclaim.constprop.0+0x98/0x338\n[   21.144763]  user_proactive_reclaim+0x2a4/0x340\n[   21.145056]  reclaim_store+0x3c/0x60\n[   21.145216]  dev_attr_store+0x20/0x40\n[   21.145585]  sysfs_kf_write+0x84/0xa8\n[   21.145835]  kernfs_fop_write_iter+0x130/0x1c8\n[   21.145994]  vfs_write+0x2b8/0x368\n[   21.146119]  ksys_write+0x70/0x110\n[   21.146240]  __arm64_sys_write+0x24/0x38\n[   21.146380]  invoke_syscall+0x50/0x120\n[   21.146513]  el0_svc_common.constprop.0+0x48/0xf8\n[   21.146679]  do_el0_svc+0x28/0x40\n[   21.146798]  el0_svc+0x34/0x110\n[   21.146926]  el0t\n---truncated---"
    }
  ],
  "lastModified": "2026-06-17T10:33:38.177",
  "configurations": [
    {
      "nodes": [
        {
          "negate": false,
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
              "vulnerable": true,
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              "vulnerable": true,
              "matchCriteriaId": "F666C8D8-6538-46D4-B318-87610DE64C34"
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            {
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              "vulnerable": true,
              "matchCriteriaId": "02259FDA-961B-47BC-AE7F-93D7EC6E90C2"
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          "operator": "OR"
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  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}