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CVE-2026-98939 json

x86/mm: Drop unnecessary PMD page copy when freeing

published 19d ago modified 19d ago also EUVD-2026-108911

cvss
—
not scored
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 5%
public exploits
0
none known

Summary

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

x86/mm: Drop unnecessary PMD page copy when freeing

On a box with a discrete GPU, lockdep reports a possible deadlock as soon as kswapd shrinks the TTM page pool. The immediate cause is an x86 commit that added an mmap_read_lock() to kernel page protection munging code.

The huge vmap code holds the same lock over a GFP_KERNEL allocation, which is a no-no now that reclaim can take it. That allocation is in a page table *free* path and ends up being for dubious purposes[1]. Basically, it tries to avoid hardware setting Accessed=1 in page table entries that are unreachable by the hardware, a non-issue.

Remove the PMD copy. Detach the original PMD page at the PUD, flush the mid-level caches, and free the PTE tables straight from the detached PMD page. With no allocation left, the locking issue is gone.

Lockdep splat/analysis:

WARNING: possible circular locking dependency detected 7.3.0-rc3-f6e7b42bf05b+ #183 Tainted: G U ------------------------------------------------------ kswapd0/269 is trying to acquire lock: ((init_mm).mmap_lock){++++}-{4:4}, at: change_page_attr_set_clr+0x29a/0x4a0 but task is already holding lock: (pool_shrink_rwsem){.+.+}-{4:4}, at: ttm_pool_shrink+0xb2/0x330 [ttm] Chain exists of: (init_mm).mmap_lock --> fs_reclaim --> pool_shrink_rwsem

The cycle is built from three edges:

1) pool_shrink_rwsem -> (init_mm).mmap_lock

The TTM shrinker restores the caching attribute of every page it frees, while holding pool_shrink_rwsem:

ttm_pool_shrink() -> ttm_pool_dispose_list() -> ttm_pool_free_page() -> set_pages_wb() -> change_page_attr_set_clr() [ init_mm mmap read lock ]

2) fs_reclaim -> pool_shrink_rwsem

The same shrinker, called from reclaim.

3) (init_mm).mmap_lock -> fs_reclaim

ioremap() installing a huge PUD mapping over an existing PMD table:

ioremap_page_range() -> vmap_range_noflush() -> vmap_try_huge_pud() [ init_mm mmap read lock ] -> pud_free_pmd_page() -> __get_free_page(GFP_KERNEL) [ enters reclaim ]

[ dhansen: Lots of changelog munging/trimming and merged comments from my version of the fix. ]

Patch / fix

  • Linux Linux 6.18.56, 7.2.10
  • linux Debian 14 7.2.10-1

debian advisories DEBIAN-CVE-2026-98939

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-98939 updated cvss 4.7

kernel: kernel: Denial of Service via circular locking dependency during page freeing

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more Red Hat Enterprise Linux 10 kernel Red Hat OpenShift Container Platform 4 openshift/ose-rhel-coreos-8, openshift/ose-rhel-coreos-9 Red Hat OpenShift Container Platform 5 openshift/ose-rhel-coreos-9
    affected: no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.17% chance of exploitation in the next 30 days, higher than 5% of all scored CVEs (FIRST EPSS, 2026-11-11).

Affected products

  • Linux Linux

Classification

Technology feeds

References

Sources

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