[CVE-2026-98547][LOW] af_packet: fix integer overflow in prb_calc_retire_blk_tmo()

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

CVE: CVE-2026-98547
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: af_packet: fix integer overflow in prb_calc_retire_blk_tmo()
Commit: 02afd75649a4e001031ed82153a5c39312ec0737
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=02afd75649a4e001031ed82153a5c39312ec0737
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98547
Analysis date: Sat, 24 Oct 2026 02:44:06 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A CAP_NET_RAW actor can configure a large TPACKET_V3 ring that overflows timeout arithmetic and drives an indefinitely restarting soft hrtimer, causing severe CPU exhaustion without memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-98547 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98547

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
======================================================================

CVE-2026-98547	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98547 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-190;**CWE-400;*CWE-835;BEST CVSS score: '5.5';DESCR 'A 32 bit integer overflow in prb_calc_retire_blk_tmo can convert an attacker controlled TPACKET_V3 block size into a negative retirement timeout. The negative interval makes the soft hrtimer expire immediately and restart at hrtimer resolution while repeatedly taking sk_receive_queue lock, so one ring can continuously consume a CPU and multiple rings can make the host unavailable. For the CVSS the PR:L value is selected because reliable triggering requires local execution with CAP_NET_RAW, which may be available to a container using host networking but is not equivalent to unauthenticated remote access. The demonstrated primitive is deterministic CPU exhaustion with C:N, I:N, and A:H. There is no evidence of an out of bounds access, use after free, information leak, or write primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 4.4 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	REMOTE PACKET KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; no additional technically supported primitive.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel-level strong DoS: the negative timeout creates a maximum-rate, indefinitely restarting soft hrtimer; multiple rings can exhaust CPUs and make the host unavailable.
* +1 Common packet-processing facility: AF_PACKET/TPACKET_V3 is a broadly available networking data path.
* -1 Reduced/delegated privilege: the complete demonstrated trigger requires CAP_NET_RAW, which is below host-administrator-equivalent authority but is not ordinary unprivileged access.
* No separate availability point: CPU exhaustion and host unavailability are the same demonstrated DoS.
* No timing penalty: triggering is deterministic, not a narrow race.
* No corruption points: the patch shows integer arithmetic overflow and a bad timeout, not OOB access, UAF, or a kernel write primitive.
* NEW52/B5-R does not apply: no retry-accumulated heap OOB write or later consumption of corrupted structured state exists.
* No confidentiality, integrity, authorization-bypass, or privilege-escalation effect is shown.

3. Reachability analysis

A local actor with CAP_NET_RAW can create the AF_PACKET socket and configure the attacker-selected TPACKET_V3 block size. CAP_NET_RAW can be delegated to a non-host-admin process or host-network container, so host-admin-equivalent authority and host UID 0 are not required. The evidence does not establish that the complete trigger works solely with attacker-obtainable user/net-namespace capabilities; therefore namespace scope remains unknown.

The demonstrated setup also needs a susceptible link-speed calculation, such as a 1 Gbps NIC, and enough memory for the large ring. This is not special or rare hardware. No race is involved, though resource limits may constrain the number of rings.

4. Primitive and severity reading

The exact primitive is attacker-influenced 32-bit integer overflow producing a negative relative hrtimer interval, followed by an infinite maximum-frequency timer restart loop while taking a socket lock. This is CLASS_C availability behavior, not memory corruption.

The shown effect is deterministic CPU exhaustion and potentially host-wide denial of service with multiple rings. There is no useful primitive past the DoS, no protected-state modification, and no unresolved trust-boundary corruption. ImportantCandidate is therefore NO, and ordinary Moderate downgrade remains allowed.

5. One-sentence report phrase

A CAP_NET_RAW actor can configure a large TPACKET_V3 ring that overflows timeout arithmetic and drives an indefinitely restarting soft hrtimer, causing severe CPU exhaustion without memory corruption.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm product-specific CAP_NET_RAW delegation, host-network container exposure, memory limits, and affected NIC speeds.

7. Call-site confidence

Call-site confidence: high

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

Patch: af_packet: fix integer overflow in prb_calc_retire_blk_tmo()
Commit: 02afd75649a4e001031ed82153a5c39312ec0737
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=02afd75649a4e001031ed82153a5c39312ec0737

Commit description:

prb_calc_retire_blk_tmo() computes in 32-bit int arithmetic:

        mbits = (blk_size_in_bytes * 8) / (1024 * 1024);

If I'm reading the validation right, tp_block_size is user
controlled and packet_set_ring() only rejects values that are <= 0
as int or not page aligned, so a 256MiB block goes right through
(and alloc_one_pg_vec_page() even has a vzalloc fallback for it).
0x10000000 * 8 wraps to INT_MIN, and on a NIC reporting 1 Gbps
(div == 1) the function ends up returning -2047.

The condition is actually (8 * size) mod 2^32 >= 2^31 && div == 1,
so the trigger set is [256,512), [768,1024), [1280,1536) and
[1792,2048) MiB. Other sizes wrap to non-negative values and faster
links divide the unsigned value back below 2^31, which is why this
doesn't blow up for everyone.

What makes it fatal is what happens next in init_prb_bdqc():

        p1->interval_ktime = ms_to_ktime(prb_calc_retire_blk_tmo(...));
        hrtimer_start(&p1->retire_blk_timer, p1->interval_ktime,
                      HRTIMER_MODE_REL_SOFT);

A negative relative timeout expires immediately. The callback
unconditionally returns HRTIMER_RESTART, and hrtimer_forward() turns
the negative interval into hrtimer_resolution:

        if (interval < hrtimer_resolution)
                interval = hrtimer_resolution;

So the SOFT timer re-fires at the maximum rate forever, holding
sk_receive_queue.lock each pass. One CPU spins in softirq until the
socket is closed. Repeat with more rings and the machine is gone.

The overflow itself is ancient - it was introduced together with
TPACKET_V3 in f6fb8f1 ("af-packet: TPACKET_V3 flexible buffer
implementation."). Its effect prior to f7460d2 ("net:
af_packet: Use hrtimer to do the retire operation", v6.18) was not
as clear-cut, though: the return value was stored into an unsigned
short retire_blk_tov, so a negative result was truncated, and a
0-jiffy delay loop could be programmed as well. Neither is nearly
as detrimental as the immediate maximum-rate spin the hrtimer
conversion turned it into.

(Unrelated to CVE-2019-20812 - that one was the ethtool failure path
returning 0, which now returns DEFAULT_PRB_RETIRE_TOV.)

Reproducer, needs CAP_NET_RAW (a --network host container has it by
default) and a 1 Gbps NIC (QEMU e1000 works):

        int fd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
        bind(fd, ...);
        int v = TPACKET_V3;
        setsockopt(fd, SOL_PACKET, PACKET_VERSION, &v, sizeof(v));
        struct tpacket_req3 req = {
                .tp_block_size = 0x10000000,
                .tp_block_nr = 1,
                .tp_frame_size = 2048,
                .tp_frame_nr = 0x10000000 / 2048,
                .tp_retire_blk_tov = 0,
        };
        setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &req, sizeof(req));

Compute in 64 bits instead. The operands are already bounded by the
existing validation, so nothing else changes. If you'd prefer a
different fix, just say so and I'll respin.

Fixes: f6fb8f1 ("af-packet: TPACKET_V3 flexible buffer implementation.")
Cc: stable@vger.kernel.org
Signed-off-by: Dairui Zhang <zhangdairui@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260923050101.1510064-1-zhangdairui@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/packet/af_packet.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=02afd75649a4e001031ed82153a5c39312ec0737

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

The original Linux kernel CVE announcement for CVE-2026-98547 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98547

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  Paranoid ActionableScore: 1

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).