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 walkpudrange() 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 numamaps of a process while VFIO-PCI is setting up DMA (specifically the vfiopinpagesremote 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:walkpgdrange+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> __walkpagerange+0x195/0x1b0 walk_pagevma+0x62/0xc0 shownumamap+0x12b/0x3b0 seqreaditer+0x297/0x440 seqread+0x11d/0x140 vfsread+0xc2/0x340 ksysread+0x5f/0xe0 dosyscall64+0x68/0x130 ? getpagefromfreelist+0x5c2/0x17e0 ? masstoreprealloc+0x17e/0x360 ? vmasetpageprot+0x4c/0xa0 ? __allocpagesnoprof+0x14e/0x2d0 ? __modmemcglruvec_state+0x8d/0x140 ? __lruvecstatmod_folio+0x76/0xb0 ? __foliomodstat+0x26/0x80 ? doanonymouspage+0x705/0x900 ? __handlemmfault+0xa8d/0x1000 ? _countmemcgevents+0x53/0xf0 ? handlemmfault+0xa5/0x360 ? douseraddrfault+0x342/0x640 ? archexittousermodeprepare.constprop.0+0x16/0xa0 ? irqentryexittousermode+0x24/0x100 entrySYSCALL64afterhwframe+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 ORIGRAX: 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 walkpmdrange() using a stable snapshot (pudpget()). If the PUD is not present or is a leaf, retry the walk via ACTIONAGAIN instead of descending further. This mirrors the retry logic in walkpterange(), which lets walkpmdrange() retry if the PTE is not being got by pteoffsetmap_lock().