[CVE-2026-68167][LOW] btrfs: do not try compression for data reloc inodes

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

CVE: CVE-2026-68167
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: btrfs: do not try compression for data reloc inodes
Commit: 31a62e4ad66313cf1ebaa00c2a17d644a4b87d22
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31a62e4ad66313cf1ebaa00c2a17d644a4b87d22
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68167
Analysis date: Mon, 10 Aug 2026 10:06:36 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
Btrfs data relocation inodes could incorrectly create compressed inline extents during writeback, causing relocation consistency checks to fail and potentially leading to a local filesystem abort or denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-68167 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68167

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-68167	LOW	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-703;CWE-754;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'btrfs data relocation inodes could be compressed during writeback and this can create an inline extent where relocation code expects normal extent mapping. The inconsistent extent state can later be detected in get_new_location and may abort relocation or place the filesystem into an error state. For the CVSS the PR:L is used in the paranoid score only for environments where a local untrusted user can trigger btrfs relocation or mount test images through delegated storage administration. The issue is not network reachable and is not triggered by packet traffic. Impact is denial of service through filesystem error or abort. No UAF, OOB access, information leak, or write primitive is indicated by the code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-68167 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	DANGER DISK SKIP IMPROVEONLY SYZBOT 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

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

## 2. Signal breakdown

* +1 Reliable DoS / filesystem abort: the bug can trigger a Btrfs error path and abort relocation or place the filesystem into an error state.
* +1 Important filesystem/storage path: this is in Btrfs relocation / extent handling, which is storage-integrity-sensitive code.
* +1 Availability impact realistic: impact is availability-oriented, mainly relocation failure or filesystem abort.
* -2 Requires admin/root/CAP_SYS_ADMIN in typical deployments: practical triggering usually requires mounting Btrfs images, running balance/relocation, or otherwise controlling filesystem administration.
* -1 Special conditions: the failure depends on data relocation, compression, writeback timing or memory pressure, and a specific inline-extent condition.

No signals for UAF, double-free, OOB write, arbitrary write, page-cache corruption, cross-file CoW bypass, information disclosure, or privilege escalation are supported by this patch.

## 3. Reachability analysis

The trigger is local and filesystem-control-plane based. In normal deployments, an attacker would need privileges to mount or manage a Btrfs filesystem and initiate relocation-like activity. Namespace or container delegation could reduce practical privileges if a container or service is allowed to mount Btrfs images or run storage management operations, but that is not the common default. The path is not network reachable and is not triggered by packet processing.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like filesystem DoS, not an Important-class kernel vulnerability. The patch prevents compression for data relocation inodes so that inline extents are not created where relocation code expects regular extent mappings. The demonstrated consequence is a Btrfs consistency failure or abort, not memory corruption with attacker-controlled reclaim, not an ownership/COW bypass, and not a plausible LPE primitive.

## 5. One-sentence report phrase

Btrfs data relocation inodes could incorrectly create compressed inline extents during writeback, causing relocation consistency checks to fail and potentially leading to a local filesystem abort or denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the issue appears to be privileged local filesystem DoS only, with no concrete memory corruption, privilege escalation, information leak, or cross-boundary write primitive indicated by the patch.

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

Patch: btrfs: do not try compression for data reloc inodes
Commit: 31a62e4ad66313cf1ebaa00c2a17d644a4b87d22
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=31a62e4ad66313cf1ebaa00c2a17d644a4b87d22

Commit description:

[BUG]
There is a syzbot report that the check inside get_new_location()
triggered:

  BTRFS info (device loop0): found 31 extents, stage: move data extents
  BTRFS info (device loop0): leaf 8908800 gen 16 total ptrs 28 free space 1676 owner 18446744073709551607
         item 0 key (256 INODE_ITEM 0) itemoff 3835 itemsize 160
                 inode generation 5 transid 0 size 0 nbytes 0
                 block group 0 mode 40755 links 1 uid 0 gid 0
                 rdev 0 sequence 0 flags 0x0
                 atime 1669132761.0
                 ctime 1669132761.0
                 mtime 1669132761.0
                 otime 0.0
         item 1 key (256 INODE_REF 256) itemoff 3823 itemsize 12
                 index 0 name_len 2
         item 2 key (258 INODE_ITEM 0) itemoff 3663 itemsize 160
                 inode generation 1 transid 16 size 733184 nbytes 106496
                 block group 0 mode 100600 links 0 uid 0 gid 0
                 rdev 0 sequence 24 flags 0x18
         item 3 key (258 EXTENT_DATA 0) itemoff 3595 itemsize 68
                 generation 16 type 0
                 inline extent data size 47 ram_bytes 4096 compression 1
  [...]
         item 27 key (18446744073709551611 ORPHAN_ITEM 258) itemoff 2376 itemsize 0
  BTRFS error (device loop0): unexpected non-zero offset in file extent item for data reloc inode 258 key offset 0 offset 9277520992061368337
  ------------[ cut here ]------------
  btrfs_abort_should_print_stack(__error)

[CAUSE]
The above dump tree shows the first file extent item is inlined, which
should make no sense for data reloc inodes, as such inodes just
represent where the data extents are in the relocation destination chunk.

However the relocation path preallocates space for each block,
then dirties them, cluster by cluster.
It's possible to have a single block at the beginning of the block
group, and no other block in the same cluster.

So relocation will preallocate a file extent for that block and dirty
the first block.  Then memory pressure forces the data reloc inode to be
written back, before any other blocks are dirtied/allocated.

Finally commit 3eaf5f0 ("btrfs: extract inlined creation into a dedicated
delalloc helper") changed the sequence of delalloc. Before that commit we
always tried NOCOW first, so that dirtied block would be written back into
the preallocated space, and appear as a regular extent.

But with that commit, we always try inline first, and since compression
is forced, we try compressing the first block, and then inline the
compressed data, resulting in the above inlined file extent in the data
reloc tree.

Then the check in get_new_location() will check the file offset, without
checking if the file extent is inlined or not, resulting in the above
failure.

[FIX]
Do not allow compression for data reloc inodes.

Since data reloc inode sizes are always block aligned, as long as we do
not compress, @data_len will always be at least one block, and
that will cause can_cow_file_range_inline() to return false, thus no
inlined extent will be created.

Reported-by: syzbot+d950c6ba09b79f6e1864@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-btrfs/6a373dc5.764cf64f.168fbe.0001.GAE@google.com/
Fixes: 3eaf5f0 ("btrfs: extract inlined creation into a dedicated delalloc helper")
CC: stable@vger.kernel.org
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/btrfs/btrfs_inode.h

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=31a62e4ad66313cf1ebaa00c2a17d644a4b87d22

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

The original Linux kernel CVE announcement for CVE-2026-68167 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68167

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:H/PR:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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).