net/sched: fq: add overflow bounds to quantum and initial quantum
fq_init() computes quantum = 2 * psched_mtu() and initial_quantum = 10 *
psched_mtu() with no overflow check. A device with a huge MTU (e.g. dummy
with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return
0x80000000; the 2 * and 10 * multiplications wrap to 0 in 32-bit
arithmetic, so q->quantum == 0. Then in fq_dequeue() the credit-refill
loop adds 0 to f->credit (which stays <= 0) and goto begin loops
forever under the qdisc lock, creating a soft lockup.
Clamp psched_mtu() to [1, 1 << 20] before multiplying so the product
cannot wrap, then cap the result at 1 << 20, matching the bound already
enforced on TCA_FQ_QUANTUM in fq_change().
Conditions to recreate the bug: a device whose MTU (plus
hard_header_len) is large enough that 2 * psched_mtu() wraps (e.g. a
dummy device with max_mtu == 0 accepting MTU 2147483634). Requires
CAP_NET_ADMIN in a user namespace.
Analysis
Analysis and contextual insights are available on OpenCVE Cloud.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
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| Linux | Linux | affected |
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No data.
No data.
No data available yet.
No vendor fix or workaround currently provided.
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Thu, 17 Sep 2026 16:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In the Linux kernel, the following vulnerability has been resolved: net/sched: fq: add overflow bounds to quantum and initial quantum fq_init() computes quantum = 2 * psched_mtu() and initial_quantum = 10 * psched_mtu() with no overflow check. A device with a huge MTU (e.g. dummy with max_mtu == 0 accepting MTU 2147483634) makes psched_mtu() return 0x80000000; the 2 * and 10 * multiplications wrap to 0 in 32-bit arithmetic, so q->quantum == 0. Then in fq_dequeue() the credit-refill loop adds 0 to f->credit (which stays <= 0) and goto begin loops forever under the qdisc lock, creating a soft lockup. Clamp psched_mtu() to [1, 1 << 20] before multiplying so the product cannot wrap, then cap the result at 1 << 20, matching the bound already enforced on TCA_FQ_QUANTUM in fq_change(). Conditions to recreate the bug: a device whose MTU (plus hard_header_len) is large enough that 2 * psched_mtu() wraps (e.g. a dummy device with max_mtu == 0 accepting MTU 2147483634). Requires CAP_NET_ADMIN in a user namespace. | |
| Title | net/sched: fq: add overflow bounds to quantum and initial quantum | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-09-17T16:05:55.517Z
Reserved: 2026-09-11T19:38:34.785Z
Link: CVE-2026-90076
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-09-17T17:16:56.820
Modified: 2026-09-17T17:16:56.820
Link: CVE-2026-90076
Redhat
No data.
OpenCVE Enrichment
No data.
No weakness.