Linux Kernel qdisc_calculate_pkt_len Amplifies User-Supplied Size Table Values Without Bounds Checking, Allowing a Crafted Size Table to Cause a Soft Lockup in the Packet Scheduler

Status: NEW  |  Advisory ID: CVE-2026-90058

Key Details

What to Know

In the Linux kernel, the following vulnerability has been resolved:

net/sched: bound qdisc_pkt_len to prevent qdisc soft lockup

qdisc_get_stab() accepts a user-supplied size table, and

__qdisc_calculate_pkt_len() amplifies qdisc_pkt_len() through the

overhead, the size-table data (u16), and size_log (up to

STAB_SIZE_LOG_MAX). A crafted stab can therefore set qdisc_pkt_len()

to ~1 GiB for an ordinary skb. Per-flow deficit schedulers such as

DRR and ETS replenish one quantum per loop iteration; with a tiny

quantum (1) they spin billions of times under the qdisc lock,

producing a soft lockup / RCU stall as illustrated by vega@nebusec.ai.

Cap the final qdisc_pkt_len() to QDISC_PKT_LEN_MAX so the size-table

amplification cannot drive deficit schedulers into an unbounded loop.

A legitimate size table (e.g. qfq's overhead 999999999, which is

handled by dropping) is still accepted.

Introduce cap QDISC_PKT_LEN_MAX (1 << 20) = 1 MiB which is well above

any legitimate single-skb wire length: the largest current skb->len

is GSO_MAX_SIZE (524280), and an ATM-style size table (53/48 cell tax)

amplifies that to ~578 KB, both comfortably below 1 MiB. At the same

time, 1 MiB bounds the deficit refill loop to ~1M iterations per

packet with quantum=1, which completes in a few milliseconds well

under the demonstrated softlockup threshold (~10^9 iterations).

Conditions to recreate the bug:

- CONFIG_NET_SCHED=y, CONFIG_NET_SCH_DRR=y (or CONFIG_NET_SCH_ETS=y).

- Attach a DRR (or ETS) root qdisc with a crafted TCA_STAB that

amplifies qdisc_pkt_len to ~1 GiB (e.g. size_log=15, data=[32768]).

- Add a class with a tiny quantum of 1 and send one small packet; the

deficit loop spins billions of times under the qdisc lock and trips

the softlockup detector (panic with kernel.softlockup_panic=1).

- Reachable as root or from an unprivileged user in a fresh user+net

namespace (unshare -Urn) with namespace-local CAP_NET_ADMIN. (NVD)

References