# CVE-2026-31456: mm/pagewalk: fix race between concurrent split and refault

Status: published. Published: 2026-04-22. Updated: 2026-11-05.

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

mm/pagewalk: fix race between concurrent split and refault

The splitting of a PUD entry in walk_pud_range() can race with a
concurrent thread refaulting the PUD leaf entry causing it to try walking
a PMD range that has disappeared.

An example and reproduction of this is to try reading numa_maps of a
process while VFIO-PCI is setting up DMA (specifically the
vfio_pin_pages_remote call) on a large BAR for that process.

This will trigger a kernel BUG:
vfio-pci 0000:03:00.0: enabling device (0000 -> 0002)
BUG: unable to handle page fault for address: ffffa23980000000
PGD 0 P4D 0
Oops: Oops: 0000 [#1] SMP NOPTI
...
RIP: 0010:walk_pgd_range+0x3b5/0x7a0
Code: 8d 43 ff 48 89 44 24 28 4d 89 ce 4d 8d a7 00 00 20 00 48 8b 4c 24
28 49 81 e4 00 00 e0 ff 49 8d 44 24 ff 48 39 c8 4c 0f 43 e3 <49> f7 06
   9f ff ff ff 75 3b 48 8b 44 24 20 48 8b 40 28 48 85 c0 74
RSP: 0018:ffffac23e1ecf808 EFLAGS: 00010287
RAX: 00007f44c01fffff RBX: 00007f4500000000 RCX: 00007f44ffffffff
RDX: 0000000000000000 RSI: 000ffffffffff000 RDI: ffffffff93378fe0
RBP: ffffac23e1ecf918 R08: 0000000000000004 R09: ffffa23980000000
R10: 0000000000000020 R11: 0000000000000004 R12: 00007f44c0200000
R13: 00007f44c0000000 R14: ffffa23980000000 R15: 00007f44c0000000
FS:  00007fe884739580(0000) GS:ffff9b7d7a9c0000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffa23980000000 CR3: 000000c0650e2005 CR4: 0000000000770ef0
PKRU: 55555554
Call Trace:
 <TASK>
 __walk_page_range+0x195/0x1b0
 walk_page_vma+0x62/0xc0
 show_numa_map+0x12b/0x3b0
 seq_read_iter+0x297/0x440
 seq_read+0x11d/0x140
 vfs_read+0xc2/0x340
 ksys_read+0x5f/0xe0
 do_syscall_64+0x68/0x130
 ? get_page_from_freelist+0x5c2/0x17e0
 ? mas_store_prealloc+0x17e/0x360
 ? vma_set_page_prot+0x4c/0xa0
 ? __alloc_pages_noprof+0x14e/0x2d0
 ? __mod_memcg_lruvec_state+0x8d/0x140
 ? __lruvec_stat_mod_folio+0x76/0xb0
 ? __folio_mod_stat+0x26/0x80
 ? do_anonymous_page+0x705/0x900
 ? __handle_mm_fault+0xa8d/0x1000
 ? __count_memcg_events+0x53/0xf0
 ? handle_mm_fault+0xa5/0x360
 ? do_user_addr_fault+0x342/0x640
 ? arch_exit_to_user_mode_prepare.constprop.0+0x16/0xa0
 ? irqentry_exit_to_user_mode+0x24/0x100
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7fe88464f47e
Code: c0 e9 b6 fe ff ff 50 48 8d 3d be 07 0b 00 e8 69 01 02 00 66 0f 1f
84 00 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00
   f0 ff ff 77 5a c3 66 0f 1f 84 00 00 00 00 00 48 83 ec 28
RSP: 002b:00007ffe6cd9a9b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007fe88464f47e
RDX: 0000000000020000 RSI: 00007fe884543000 RDI: 0000000000000003
RBP: 00007fe884543000 R08: 00007fe884542010 R09: 0000000000000000
R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000
 </TASK>

Fix this by validating the PUD entry in walk_pmd_range() using a stable
snapshot (pudp_get()).  If the PUD is not present or is a leaf, retry the
walk via ACTION_AGAIN instead of descending further.  This mirrors the
retry logic in walk_pte_range(), which lets walk_pmd_range() retry if the
PTE is not being got by pte_offset_map_lock().

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ f9e54c3a2f5b79ecc57c7bc7d0d3521e461a2101; < 138ada1337b47cc5efb08e517674d46f86268274 (affected, git); ≥ f9e54c3a2f5b79ecc57c7bc7d0d3521e461a2101; < 38ec58670a0c5fc1edabdeccd857e586b7b3f318 (affected, git); ≥ f9e54c3a2f5b79ecc57c7bc7d0d3521e461a2101; < 9bbbebd94dd5be25ec8c899d46ef01b33d5d22c0 (affected, git); ≥ f9e54c3a2f5b79ecc57c7bc7d0d3521e461a2101; < 3b89863c3fa482912911cd65a12a3aeef662c250 (affected, git). Source: CNA: Linux
- Linux / Linux: 6.12 (affected); ≥ 0; < 6.12 (unaffected, semver); ≥ 6.12.84; ≤ 6.12.* (unaffected, semver); ≥ 6.18.21; ≤ 6.18.* (unaffected, semver); ≥ 6.19.11; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.09% as of 2026-11-05; percentile 0.39%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/138ada1337b47cc5efb08e517674d46f86268274 (CNA: Linux)

- https://git.kernel.org/stable/c/38ec58670a0c5fc1edabdeccd857e586b7b3f318 (CNA: Linux)

- https://git.kernel.org/stable/c/9bbbebd94dd5be25ec8c899d46ef01b33d5d22c0 (CNA: Linux)

- https://git.kernel.org/stable/c/3b89863c3fa482912911cd65a12a3aeef662c250 (CNA: Linux)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-31456; retrieved 2026-11-05; source updated 2026-11-05

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-31456; retrieved 2026-11-05; source updated Not reported

Revision: 11904fce095a584df42628b1b670fdc89fa7bc21a87faabe245d8d473199e8e8

Canonical: https://y2.dev/cve/CVE-2026-31456

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