CVE-2026-31456

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().