CVE-2026-90058net/sched: bound qdisc_pkt_len to prevent qdisc soft lockup

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/4f0f4b09a95ef602fbdae948326beadd9c20eab3
• https://git.kernel.org/stable/c/0bc5a6280a132a2170c6fa742436e01b635f02de
• https://git.kernel.org/stable/c/448c34421b38ec7c93a0060a885218ca319d382b
• https://git.kernel.org/stable/c/82fdbf49b9f7528c82bf608cfa9710e5f81a198c
• https://git.kernel.org/stable/c/6e78792d8ce1fa3561713860ee8086901dde4f99
• https://git.kernel.org/stable/c/1a5c26e586481bc82abc19ac92c3d9f10518f525
• https://git.kernel.org/stable/c/16d21a27df3f50b7251c9388fc5a7ab22703b469
• https://git.kernel.org/stable/c/8f735d64382dcf162f4276d6699d03ad2f859c0b