EUVD-2026-76666
GHSA-6j58-c7p9-8cjq
Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
AV:L and PR:L match the local UID-1000 trigger; AC:H reflects the race condition between PMD clearance and walk retry that must align for the stale state to persist.
Primary rating from Vendor (Linux).
CVSS VectorVendor: Linux
Lifecycle Timeline
5DescriptionCVE.org
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 ---truncated---
AnalysisAI
Stale walk->action state in the Linux kernel mm/pagewalk subsystem causes walk_pud_range() to spuriously retry PUD traversal and invoke page-walk callbacks twice, resulting in a slab-out-of-bounds write reachable by unprivileged local users via the mincore(2) syscall. Affected kernels in the 6.18.21-6.18.49 and 7.0-7.2.3 ranges contain the latent logic error activated by commit 3b89863c3fa4; the bug was confirmed by KASAN output and a local fuzzer producing an OOB write into a sigqueue slab object. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires: (1) a local user account - standard unprivileged access suffices (UID 1000 confirmed in KASAN trace, CVSS PR:L); (2) a kernel version in the range 6.18.21-6.18.49 or 7.0-7.2.3 containing commit 3b89863c3fa4 but not the fix; (3) the ability to invoke the mincore(2) syscall, which is permitted by default for unprivileged users on most Linux distributions; (4) the ability to cause a PMD entry to be cleared concurrently with the walk_pmd_range() ACTION_AGAIN retry - this is the race window that must be hit. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The NVD CVSS of 7.8 (AV:L/AC:L/PR:L) captures the local, low-privilege nature of the bug correctly, but AC:L understates the race condition between PMD clearance and the walk retry path that must align for the stale action to persist. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Upgrade to Linux kernel 6.18.50, 7.2.4, or 7.3-rc1, all of which contain the fix that resets walk->action to ACTION_SUBTREE when a cleared PMD is encountered during an ACTION_AGAIN retry. … Detailed patch versions, workarounds, and compensating controls in full report. |
Recommended ActionAI
Within 24 hours, conduct an inventory of all Linux systems running kernels in the 6.18.21-6.18.49 or 7.0-7.2.3 ranges; use kernel version checks via uname -r or automated asset management tools to identify affected instances. …
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