[CVE-2026-74590][MODERATE REGULAR] fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions

[CVE-2026-74590][MODERATE REGULAR] fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions

[AL-KERNEL]

CVE: CVE-2026-74590
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions
Commit: 1344b632cb5043e32939a84568125719111c5af3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1344b632cb5043e32939a84568125719111c5af3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74590
Analysis date: Sat, 22 Aug 2026 14:05:22 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended / Actionable Moderate candidate
Manual review required: YES

Summary:
`bpf_get_fsverity_digest() used a mutable dynptr field as a memset offset, so concurrent modification of arg digest_size could trigger an attacker-influenced out-of-bounds zero write and kernel crash from a privileged or delegated BPF context.`

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74590 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74590

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: MODERATE REGULAR
Manual review required: YES

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-2026-74590	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;CWE-367;*CWE-787;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'bpf_get_fsverity_digest() can violate dynptr memory safety because it used the mutable arg digest_size field as the offset for zero filling after copying the fsverity digest. A concurrent BPF context can modify that field while the kfunc is running, so memset may start at an attacker influenced offset outside the intended dynptr buffer. This is a local race condition with an out-of-bounds zero write candidate and at minimum can crash the kernel. For the CVSS the PR:L is used in the paranoid score because some deployments may delegate BPF program loading to non root service accounts or reduced capability users, while the base score keeps PR:H for typical privileged BPF access. The issue is not network reachable. Impact is at least local denial of service and in the worst still defensible interpretation may include confidentiality and integrity impact due to kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate candidate (with actual score 3)	YES	DISK BPF SIMPLEFIX LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate candidate**

## 2. Signal breakdown

Paranoid scoring signals:

* Constrained kernel buffer overwrite: +1. The old code used mutable `arg->digest_size` as the destination offset for `memset()`, while the length was based on `hash_alg->digest_size`, so concurrent modification can move the zero-fill start outside the intended dynptr layout.
* Weak LPE concern: +1. This is not a demonstrated privilege-escalation primitive, but an attacker-influenced offset in a kernel `memset()` is enough to keep it in manual-review scope.
* Reliable kernel crash / strong DoS: +1. The commit explicitly states that the bug could crash if `arg->digest_size` is concurrently modified.
* Integrity impact plausible: +1. The corruption is a zero-write rather than attacker-controlled data, but it can still corrupt adjacent kernel memory or dynptr-backed state.
* Availability impact realistic: +1. A kernel crash from a BPF kfunc path is host-wide availability impact.
* Requires privileged BPF access in typical deployments: -1 paranoid, -2 conservative. Conservative scoring assumes root or CAP_BPF/CAP_SYS_ADMIN gating. Paranoid scoring treats delegated BPF access or reduced-capability service accounts as less than full host-root.

Not awarded:

* No remote/network trigger.
* No local unprivileged trigger under the stated product configuration.
* No high-control unprivileged BPF API bonus because unprivileged BPF is disabled or gated.
* No strong LPE plausibility because there is no demonstrated reclaim, object replacement, callback control, arbitrary write, or type confusion.

## 3. Reachability analysis

The bug is reachable through `bpf_get_fsverity_digest()`, a BPF kfunc tied to fsverity digest retrieval. In typical hardened distributions, loading or attaching the relevant BPF program requires root, CAP_BPF, CAP_SYS_ADMIN, or equivalent privileged policy, so the conservative interpretation is privileged local only. Namespaces or containers matter if BPF loading is delegated to container root or a service account with reduced capabilities, which supports the paranoid PR:L-like interpretation but not a true unprivileged trigger. The path is not network reachable and exploitation requires concurrent modification of the dynptr contents while the kfunc is executing.

Call-site confidence: medium. The affected function body and the immediate corruption sink are visible, but broader caller and program-type constraints are not fully shown in the supplied patch.

## 4. Severity interpretation

This behaves above an ordinary low-risk crash because the patch fixes a memory-safety violation in a BPF kfunc and the sink is an attacker-influenced `memset()` offset. Realistic exploitation is constrained by privileged BPF access and by the race requirement, and the overwrite payload is fixed zeroes rather than attacker-chosen data. Therefore it is not a Strong Important candidate on the supplied evidence, but it should not be auto-closed as a trivial DoS.

Theoretical risk: local kernel memory corruption and possible LPE concern in the paranoid case.
Realistic evidence: local privileged or delegated-BPF crash/corruption scenario, with no demonstrated exploit primitive.

## 5. One-sentence report phrase

`bpf_get_fsverity_digest() used a mutable dynptr field as a memset offset, so concurrent modification of arg digest_size could trigger an attacker-influenced out-of-bounds zero write and kernel crash from a privileged or delegated BPF context.`

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is a BPF dynptr race with a plausible out-of-bounds write sink, and although the practical trigger is privilege-gated and constrained, the memory-corruption class is sufficient for manual kernel-security review.

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

Patch: fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions
Commit: 1344b632cb5043e32939a84568125719111c5af3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1344b632cb5043e32939a84568125719111c5af3

Commit description:

The BPF verifier and the dynptr abstraction ensure that the memory space
referenced by a dynptr remains valid.  They do not, however, provide any
guarantee that the contents of the memory are stable.  kfuncs are
expected to remain memory-safe even if concurrent modifications occur.

bpf_get_fsverity_digest() didn't follow that: it could crash if
arg->digest_size was concurrently modified.

Fix that by using the known-good value hash_alg->digest_size instead.

Also widen 'dynptr_sz' and 'out_digest_sz' to u64 to match the return
type of __bpf_dynptr_size().  It doesn't appear that it can actually be
more than INT_MAX currently (since __bpf_dynptr_data_rw() excludes
file-based pointers), but the correct type might as well be used.

Fixes: 67814c0 ("bpf, fsverity: Add kfunc bpf_get_fsverity_digest")
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/bpf/20260803181232.14743-2-ebiggers@kernel.org
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/verity/measure.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=1344b632cb5043e32939a84568125719111c5af3

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

The original Linux kernel CVE announcement for CVE-2026-74590 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74590

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 2
  Paranoid ActionableScore: 4

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: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).