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CVE-2026-89754

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

7.8 EPSS 0.2% AV:L

Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem mm. Debian urgency: unimportant.

What to do, per Debian release

Debian's own status for CVE-2026-89754 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source

Release	Status	Version	What this means
Debian 12bookworm	not affected	6.1.190-1	Nothing to do. bookworm never shipped the vulnerable code.
Debian 13trixie	not affected	6.12.113-1	Nothing to do. trixie never shipped the vulnerable code.
Debian 14forky	fixed	7.2.6-1	Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts.
Debian unstablesid	fixed	7.2.6-1	Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts.

The vulnerability

What the bug is

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 ---truncated---

Where it lives

The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.

mm/pagewalk.c

What this is: MEMORY MANAGEMENT - COREMAINTAINERS section matched on mm/pagewalk.c · linux-mm@kvack.org · Maintained

Show whether Debian builds this, per release

Not established. This site could not find the build rule for the affected file in the kernel trees Debian ships, so it has no answer about whether Debian compiles this code. Read that as unknown, never as absent.

bookworm, trixie, forky, sid?

?: not established - no Makefile entry for the affected file

Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.

From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS

Debian tracker CVE record NVD kernel CNA record

Signals

Published: 2026-09-11 (37 days ago)source
CVSS: 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it
Attack vector: local (AV:L)source
EPSS: 0.17%, higher than 6.3% of all CVEssource
CISA KEV: not listedthe KEV catalogue
Debian urgency: unimportantsource
Subsystem: mm

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
6.18.21	6.18.50	895cd4ecbb2e
7.0	7.2.4	c05cdc2ab365
7.0	7.3	aedf2efd1897
6.19.11	0	0

Why the CNA scored it this way

Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source

AV:LAttack vector, local: Triggered by the unprivileged mincore(2) syscall walking the caller's own page tables; other local walkers such as /proc/self/pagemap share the same pagewalk bug. The path is not reachable from network packet processing.
AC:LAttack complexity, low: The stale ACTION_AGAIN escape is a race with concurrent PMD zap, but the attacker controls both sides by calling mincore in one thread while another faults pages and issues madvise(MADV_DONTNEED) on the same mapping, which is a documented unprivileged repro.
PR:LPrivileges required, low: mincore(2) requires only a normal local user; the KASAN report was from UID 1000 with no capabilities, and neither root nor user-namespace capabilities are needed.
UI:NUser interaction, none: The attacker invokes mincore and madvise against their own mapping; no separate victim action is required.
S:UScope, unchanged: Impact is kernel heap corruption and possible privilege escalation on the same host kernel; it does not cross a VM, IOMMU, or other security-authority boundary.
C:HConfidentiality impact, high: Duplicate walk callbacks overflow mincore's kernel vector into adjacent heap (observed as a slab OOB write) and can copy an extra pagemap entry from neighboring kernel memory via /proc/self/pagemap. That memory corruption can be turned into an arbitrary kernel read.
I:HIntegrity impact, high: mincore's cursor has no bounds check, so a retried PUD walk writes past the one-page __get_free_page() buffer into adjacent kernel memory, and a fully zapped range can retry unbounded 0/1 writes. That heap corruption is exploitable for arbitrary writes and control-flow hijacking.
A:HAvailability impact, high: The out-of-bounds write can oops or panic the kernel, and leaving walk->action at ACTION_AGAIN after a zap can make walk_pud_range retry the same PUD indefinitely, hanging the CPU.