Google: sys-kernel/cchost-kernel-6_12, sys-kernel/csql-kernel-6_12: security update to 19506.120.52

medium Tenable Cloud Security Plugin ID 472123

Description

There are packages installed that are affected by a vulnerability referenced in the following CVE:

- In the Linux kernel, the following vulnerability has been resolved: btrfs: do not strictly require dirty
metadata threshold for metadata writepages [BUG] There is an internal report that over 1000 processes are
waiting at the io_schedule_timeout() of balance_dirty_pages(), causing a system hang and trigger a kernel
coredump. The kernel is v6.4 kernel based, but the root problem still applies to any upstream kernel
before v6.18. [CAUSE] From Jan Kara for his wisdom on the dirty page balance behavior first. This cgroup
dirty limit was what was actually playing the role here because the cgroup had only a small amount of
memory and so the dirty limit for it was something like 16MB. Dirty throttling is responsible for
enforcing that nobody can dirty (significantly) more dirty memory than there's dirty limit. Thus when a
task is dirtying pages it periodically enters into balance_dirty_pages() and we let it sleep there to slow
down the dirtying. When the system is over dirty limit already (either globally or within a cgroup of the
running task), we will not let the task exit from balance_dirty_pages() until the number of dirty pages
drops below the limit. So in this particular case, as I already mentioned, there was a cgroup with
relatively small amount of memory and as a result with dirty limit set at 16MB. A task from that cgroup
has dirtied about 28MB worth of pages in btrfs btree inode and these were practically the only dirty pages
in that cgroup. So that means the only way to reduce the dirty pages of that cgroup is to writeback the
dirty pages of btrfs btree inode, and only after that those processes can exit balance_dirty_pages(). Now
back to the btrfs part, btree_writepages() is responsible for writing back dirty btree inode pages. The
problem here is, there is a btrfs internal threshold that if the btree inode's dirty bytes are below the
32M threshold, it will not do any writeback. This behavior is to batch as much metadata as possible so we
won't write back those tree blocks and then later re-COW them again for another modification. This
internal 32MiB is higher than the existing dirty page size (28MiB), meaning no writeback will happen,
causing a deadlock between btrfs and cgroup: - Btrfs doesn't want to write back btree inode until more
dirty pages - Cgroup/MM doesn't want more dirty pages for btrfs btree inode Thus any process touching that
btree inode is put into sleep until the number of dirty pages is reduced. Thanks Jan Kara a lot for the
analysis of the root cause. [ENHANCEMENT] Since kernel commit b55102826d7d ("btrfs: set AS_KERNEL_FILE on
the btree_inode"), btrfs btree inode pages will only be charged to the root cgroup which should have a
much larger limit than btrfs' 32MiB threshold. So it should not affect newer kernels. But for all current
LTS kernels, they are all affected by this problem, and backporting the whole AS_KERNEL_FILE may not be a
good idea. Even for newer kernels I still think it's a good idea to get rid of the internal threshold at
btree_writepages(), since for most cases cgroup/MM has a better view of full system memory usage than
btrfs' fixed threshold. For internal callers using btrfs_btree_balance_dirty() since that function is
already doing internal threshold check, we don't need to bother them. But for external callers of
btree_writepages(), just respect their requests and write back whatever they want, ignoring the internal
btrfs threshold to avoid such deadlock on btree inode dirty page balancing. (CVE-2026-23157)

Solution

Update the sys-kernel/cchost-kernel-6_12 library and its related packages to version 19506.120.52 or later.

Plugin Details

Severity: Medium

ID: 472123

Version: Revision 1.1

Type: Local

Family: Google COS Security Checks

Published: 10/3/2026

Updated: 10/3/2026

Risk Information

VPR

Risk Factor: Low

Score: 3

Percentile: 23.51

Vendor

Vendor Severity: LOW

CVSS v2

Risk Factor: Medium

Base Score: 4.6

Temporal Score: 3.4

Vector: CVSS2#AV:L/AC:L/Au:S/C:N/I:N/A:C

CVSS Score Source: CVE-2026-23157

CVSS v3

Risk Factor: Medium

Base Score: 5.5

Temporal Score: 4.8

Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Temporal Vector: CVSS:3.0/E:U/RL:O/RC:C

Vulnerability Information

Exploit Ease: No known exploits are available

Vulnerability Publication Date: 2/14/2026

Reference Information

CVE: CVE-2026-23157

Detections

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