[CVE-2026-68150][LOW] fs/super: fix emergency thaw double-unlock of s_umount

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-68150
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fs/super: fix emergency thaw double-unlock of s_umount
Commit: c78e38745ff1b0457c4551e7f75ea15842df1169
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c78e38745ff1b0457c4551e7f75ea15842df1169
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68150
Analysis date: Mon, 10 Aug 2026 16:20:09 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
Emergency thaw used incompatible superblock locking semantics, causing a double unlock of `s_umount` that can corrupt rwsem state and produce a local privileged denial of service or system instability.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-68150 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68150

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-68150	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';CWE-667;CWE-662;CWE-703;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'Emergency thaw can corrupt the s_umount rwsem state because __iterate_supers with SUPER_ITER_EXCL already holds s_umount exclusively before calling do_thaw_all_callback, while thaw_super_locked releases the same lock on every path. A caller that can trigger emergency thaw may therefore cause a second unlock in __iterate_supers, producing DEBUG_RWSEMS warnings and possible lock state corruption. For the CVSS the PR:H is used because reliable triggering normally requires administrative control of SysRq or equivalent emergency filesystem management. The issue is not network reachable and is triggered through a local administrative control path. Impact is denial of service or system instability due to corrupted locking state, with no supported path to information disclosure or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-68150 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 2992476528aeecbaee17ba0a6396a817481205a3 YES  NO NO unknown 	MAYBE	LOCK DISK HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: **1**
* Paranoid score: **2**
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* **Availability impact realistic: +1**
  The bug can corrupt `s_umount` rwsem state and trigger a `DEBUG_RWSEMS` warning. Practical impact is local DoS or system instability, especially with stricter debug or panic-on-warn settings.

* **Important filesystem/storage path: +1**
  The affected code is in core superblock thaw handling, but this is an emergency administrative path rather than normal filesystem I/O or metadata update flow.

* **Requires admin/root/CAP_* in typical deployments: -2**
  Triggering emergency thaw normally requires SysRq or equivalent privileged filesystem/system management access.

Paranoid additional signal:

* **Reliable kernel crash / strong DoS: +1**
  Counted only for the paranoid score because the warning can become a crash with `panic_on_warn`, and corrupted rwsem state may destabilize later superblock locking. The patch does not show UAF, OOB access, arbitrary write, or attacker-controlled memory corruption.

Not awarded:

* **No local unprivileged trigger.**
* **No network reachability.**
* **No generic memory corruption primitive.**
* **No privilege escalation plausibility.**
* **No confidentiality or integrity impact.**

## 3. Reachability analysis

The trigger is local and administrative. A user needs the ability to invoke emergency thaw, typically through SysRq or an equivalent privileged control path. Containers or namespaces do not normally reduce this to an unprivileged trigger unless the environment delegates dangerous host-level emergency controls, which would be unusual.

The path is not network reachable and is not triggered by ordinary filesystem reads or writes. It is in core VFS superblock infrastructure, but the affected operation is an emergency thaw path rather than a default data-plane path.

Call-site confidence: **high**, because the patch includes both `do_thaw_all()` and `do_thaw_all_callback()`, and the lock ownership mismatch is explicit.

## 4. Severity interpretation

This behaves like an **ordinary Moderate / Low-like administrative DoS** issue. The bug is a locking ownership error: `__iterate_supers(..., SUPER_ITER_EXCL)` takes `s_umount`, while `thaw_super_locked()` also releases it, causing a double unlock.

The theoretical concern is corrupted rwsem state, but the provided patch and trace show a warning and possible instability, not a concrete UAF, double-free, OOB write, arbitrary write, or privilege escalation primitive. Realistic exploitation requires administrative access and mainly affects availability.

## 5. One-sentence report phrase

Emergency thaw used incompatible superblock locking semantics, causing a double unlock of `s_umount` that can corrupt rwsem state and produce a local privileged denial of service or system instability.

## 6. Manual review recommendation

**NO MANUAL CHECK NEEDED**

This does not show unprivileged reachability, network exposure, concrete memory corruption, privilege escalation, or confidentiality/integrity impact. It can be handled as a low-priority administrative DoS unless the product exposes SysRq or emergency thaw to reduced-privilege users.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: fs/super: fix emergency thaw double-unlock of s_umount
Commit: c78e38745ff1b0457c4551e7f75ea15842df1169
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c78e38745ff1b0457c4551e7f75ea15842df1169

Commit description:

do_thaw_all() iterates over all superblocks via __iterate_supers()
with SUPER_ITER_EXCL, which acquires s_umount exclusively before
calling the callback and releases it afterwards. However, the
callback do_thaw_all_callback() calls thaw_super_locked() which
unconditionally releases s_umount on every code path. This results
in a second unlock attempt in __iterate_supers() that corrupts the
rwsem state, triggering a DEBUG_RWSEMS warning:

[  182.601148] sysrq: Emergency Thaw of all frozen filesystems
[  182.601865] ------------[ cut here ]------------
[  182.602375] DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)): count = 0x0, magic = 0xffff99b1011e5870, owner = 0x0, curr 0xffff99b101b06c80, list not empty
[  182.603817] WARNING: kernel/locking/rwsem.c:1412 at up_write+0xa3/0x170, CPU#2: kworker/2:1/53
[  182.604578] Modules linked in:
[  182.604864] CPU: 2 UID: 0 PID: 53 Comm: kworker/2:1 Not tainted 7.2.0-rc4-00001-gbd3bd93ea98a-dirty #4 PREEMPT(lazy)
[  182.605711] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1kylin1 04/01/2014
[  182.606417] Workqueue: events do_thaw_all
[  182.606750] RIP: 0010:up_write+0xaf/0x170
[  182.607076] Code: 19 3a 92 48 0f 44 c2 48 8b 55 08 48 8b 55 00 4c 8b 45 08 48 8b 55 00 48 8d 3d ad 91 e0 01 48 8b 4d 20 50 48 c7 c6 f0 8c 26 92 <67> 48 0f b9 3a e8 d7 93 4e 00 58 eb 81 48 83 7f 18 00 48 c7 c2 8d
[  182.608563] RSP: 0018:ffffb670001d7e08 EFLAGS: 00010246
[  182.609007] RAX: ffffffff92349e8d RBX: 0000000000000000 RCX: ffff99b1011e5870
[  182.609595] RDX: 0000000000000000 RSI: ffffffff92268cf0 RDI: ffffffff92914d10
[  182.610283] RBP: ffff99b1011e5870 R08: 0000000000000000 R09: ffff99b101b06c80
[  182.610847] R10: ffff99b10139a808 R11: fefefefefefefeff R12: 0000000000000000
[  182.611414] R13: ffffffff90cf74d0 R14: 0000000000000000 R15: ffff99b1011e5800
[  182.612009] FS:  0000000000000000(0000) GS:ffff99b1eaaee000(0000) knlGS:0000000000000000
[  182.612670] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  182.613146] CR2: 00000000005c631c CR3: 00000000013ee000 CR4: 00000000000006f0
[  182.613722] Call Trace:
[  182.613946]  <TASK>
[  182.614130]  __iterate_supers+0x128/0x150
[  182.614463]  do_thaw_all+0x1b/0x30
[  182.614759]  process_scheduled_works+0xbb/0x3f0
[  182.615150]  ? __pfx_worker_thread+0x10/0x10
[  182.615499]  worker_thread+0x129/0x270
[  182.615816]  ? __pfx_worker_thread+0x10/0x10
[  182.616201]  kthread+0xe2/0x120
[  182.616469]  ? __pfx_kthread+0x10/0x10
[  182.616792]  ret_from_fork+0x15b/0x240
[  182.617115]  ? __pfx_kthread+0x10/0x10
[  182.617426]  ret_from_fork_asm+0x1a/0x30
[  182.617761]  </TASK>
[  182.617968] ---[ end trace 0000000000000000 ]---
[  182.618412] Emergency Thaw complete

Fix this by switching to SUPER_ITER_UNLOCKED and acquiring s_umount
in the callback via super_lock_excl() before calling
thaw_super_locked(). This matches the locking pattern expected by
thaw_super_locked() and eliminates the double unlock.

While at it, remove the dead 'return;' at the end of
do_thaw_all_callback().

Fixes: 2992476 ("super: use a common iterator (Part 1)")
Cc: stable@vger.kernel.org
Signed-off-by: Chen Changcheng <chenchangcheng@kylinos.cn>
Link: https://patch.msgid.link/20260721064140.152305-1-chenchangcheng@kylinos.cn
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/locking/rwsem.c
  fs/super.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c78e38745ff1b0457c4551e7f75ea15842df1169

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-68150 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68150

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.4

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 1
  Paranoid ActionableScore: 2

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).