Bug 2536275 (CVE-2026-90074)

Summary: CVE-2026-90074 kernel: net/sched: fq_pie: clamp default quantum to avoid signed overflow
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
Whiteboard:
Fixed In Version: Doc Type: ---
Doc Text:
A flaw was found in the Linux kernel's fq_pie network scheduler. A local attacker with `CAP_NET_ADMIN` capabilities in a user namespace could exploit this by configuring a network device with a specially crafted large Maximum Transmission Unit (MTU) value. This can lead to a signed integer overflow, causing an infinite loop and a system soft lockup. This vulnerability results in a Denial of Service (DoS).
 Story Points: ---
Clone Of: Environment:
Last Closed: Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-17 17:14:25 UTC 
In the Linux kernel, the following vulnerability has been resolved:

net/sched: fq_pie: clamp default quantum to avoid signed overflow

fq_pie_init() sets q->quantum = psched_mtu(qdisc_dev(sch)) without
clamping. A device with a huge MTU (e.g. dummy with max_mtu == 0
accepting MTU 2147483634) makes psched_mtu() return 0x80000000, which
overflows the signed flow->deficit to INT_MIN in fq_pie_qdisc_dequeue(),
causing an infinite loop and soft lockup. Emulate fq_pie_policy which
is already bounded to [1, 1 << 20]; clamp the default to [256, 1 << 20].
256 matches fq_codel's floor and is a sane minimum for a DRR quantum.

Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) wraps psched_mtu() into the sign bit (e.g. a dummy
device with max_mtu == 0 accepting MTU 2147483634). Requires
CAP_NET_ADMIN in a user namespace.
Comment 1 Keith Grant 2026-09-22 17:43:52 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-90074.mbox