CVE-2026-89754: mm/pagewalk: fix stale walk->action escaping walk_pmd_range()

CVE-2026-89754: mm/pagewalk: fix stale walk->action escaping walk_pmd_range()

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

mm/pagewalk: fix stale walk->action escaping walk_pmd_range()

If ->pmd_entry() sets walk->action = ACTION_AGAIN, the pmd_none() check is
retried.  The PMD entry may be cleared at the point of retry.

In this case, if walk->ops->install_pte is not specified, the code
continues to the next PMD entry in the range without resetting
walk->action to ACTION_SUBTREE.

This leaves walk->action erroneously set to ACTION_AGAIN, which is
incorrect.

This was incorrect but not problematic up until commit 3b89863c3fa4
("mm/pagewalk: fix race between concurrent split and refault") which
updated walk_pud_range() to check for walk->action == ACTION_AGAIN upon
walk_pmd_range()'s return, causing the PUD walk to be retried.

In this case this results in duplicate walk callbacks being invoked,
which is erroneous and will break any caller that is not idempotent
with respect to this (and waste time for those which are).  The result
is an out-of-bounds write, triggered by a local fuzzer:

[    2.272695] ==================================================================
[    2.273471] BUG: KASAN: slab-out-of-bounds in __mincore_unmapped_range+0x14f/0x190
[    2.274302] Write of size 1 at addr ffff888008d9b000 by task poc/106
[    2.274966]
[    2.275154] CPU: 0 UID: 1000 PID: 106 Comm: poc Not tainted 7.2.0-rc6-00429-ga7c7074b58d2 #55 PREEMPT(lazy)
[    2.275159] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[    2.275164] Call Trace:
[    2.275170]  <TASK>
[    2.275172]  dump_stack_lvl+0x53/0x70
[    2.275200]  print_report+0xd0/0x630
[    2.275210]  ? __pfx__raw_spin_lock_irqsave+0x10/0x10
[    2.275219]  ? irqentry_exit+0xd2/0x670
[    2.275224]  ? irqentry_exit+0xd2/0x670
[    2.275226]  ? __virt_addr_valid+0xef/0x1a0
[    2.275239]  ? __mincore_unmapped_range+0x14f/0x190
[    2.275242]  kasan_report+0xce/0x100
[    2.275245]  ? __mincore_unmapped_range+0x14f/0x190
[    2.275248]  __mincore_unmapped_range+0x14f/0x190
[    2.275252]  mincore_unmapped_range+0x45/0x70
[    2.275254]  walk_pgd_range+0xafc/0xfc0
[    2.275261]  ? __pfx_walk_pgd_range+0x10/0x10
[    2.275264]  ? __update_load_avg_se+0x3d1/0x670
[    2.275275]  __walk_page_range+0xc0/0x310
[    2.275278]  ? __pfx_find_vma+0x10/0x10
[    2.275281]  ? finish_task_switch.isra.0+0x16d/0x4f0
[    2.275290]  walk_page_range_mm_unsafe+0x26f/0x3a0
[    2.275293]  ? __pfx_mtree_load+0x10/0x10
[    2.275298]  ? __pfx_walk_page_range_mm_unsafe+0x10/0x10
[    2.275302]  ? __free_frozen_pages+0x54d/0x7e0
[    2.275308]  __do_sys_mincore+0x132/0x380
[    2.275311]  do_syscall_64+0xf9/0x540
[    2.275316]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[    2.275322] RIP: 0033:0x422ccd
[    2.275326] Code: b3 66 2e 0f 1f 84 00 00 00 00 00 66 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
[    2.275329] RSP: 002b:00007fffffffec18 EFLAGS: 00000287 ORIG_RAX: 000000000000001b
[    2.275337] RAX: ffffffffffffffda RBX: 0000000000000066 RCX: 0000000000422ccd
[    2.275339] RDX: 00000000004d0940 RSI: 0000000001000000 RDI: 00007ffff4000000
[    2.275340] RBP: 00000000004d0940 R08: 0000000000000100 R09: 0000000000000100
[    2.275342] R10: 0000000000000100 R11: 0000000000000287 R12: 20c49ba5e353f7cf
[    2.275343] R13: 00000000004990d3 R14: 0000000000000000 R15: 0000000000000001
[    2.275346]  </TASK>
[    2.275347]
[    2.296904] The buggy address belongs to the object at ffff888008d9b000
[    2.296904]  which belongs to the cache sigqueue of size 80
[    2.298151] The buggy address is located 0 bytes inside of
[    2.298151]  allocated 80-byte region [ffff888008d9b000, ffff888008d9b050)
[    2.299408]
[    2.299601] The buggy address belongs to the physical page:
[    2.300191] page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x8d9b
[    2.301001] flags: 0x100000000000000(node=0|zone=1)
[    2.301535] page_type: f5(slab)
[    2.301884] raw: 0100000000000000 ffff888107e46780 dead000000000122 0000000000000000
[    2.302687] raw: 0000000000000000 0000000800240024 00000000f5000000 0000000000000000
[    2.303489] page dumped because: kasan: bad access detected
[    2.304092]
[    2.304276] Memory state around the buggy address:
[    2.304801]  ffff888008d9af00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    2.305567]  ffff888008d9af80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    2.306340] >ffff888008d9b000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[    2.307115]                    ^
[    2.307474]  ffff888008d9b080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[    2.308237]  ffff888008d9b100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[    2.308997] ==================================================================

A specific example of this breaking things is mincore which walks an
internal cursor data structure a byte at a time on assumption that page
table entry callbacks are called only once for each entry.

Fix the problem by resetting walk->action to ACTION_SUBTREE prior to the
none check.

The pattern also exists in walk_pud_range() so fix it there too.

This issue was found through AI-based fuzzing.

The Linux kernel CVE team has assigned CVE-2026-89754 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.18.21 with commit 38ec58670a0c5fc1edabdeccd857e586b7b3f318 and fixed in 6.18.50 with commit 895cd4ecbb2e03d7583103c65ee5adaae126ed6d
	Issue introduced in 7.0 with commit 3b89863c3fa482912911cd65a12a3aeef662c250 and fixed in 7.2.4 with commit c05cdc2ab365b98099386e6c166b9865a4f2a9d3
	Issue introduced in 7.0 with commit 3b89863c3fa482912911cd65a12a3aeef662c250 and fixed in 7.3-rc1 with commit aedf2efd18977e0cef7eb963166e2b1fcc0aa321
	Issue introduced in 6.19.11 with commit 9bbbebd94dd5be25ec8c899d46ef01b33d5d22c0

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89754
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	mm/pagewalk.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/895cd4ecbb2e03d7583103c65ee5adaae126ed6d
	https://git.kernel.org/stable/c/c05cdc2ab365b98099386e6c166b9865a4f2a9d3
	https://git.kernel.org/stable/c/aedf2efd18977e0cef7eb963166e2b1fcc0aa321