CVE-2025-38115: net_sched: sch_sfq: fix a potential crash on gso_skb handling

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

netsched: schsfq: fix a potential crash on gsoskb handling

SFQ has an assumption of always being able to queue at least one packet.

However, after the blamed commit, sch->q.len can be inflated by packets in sch->gsoskb, and an enqueue() on an empty SFQ qdisc can be followed by an immediate drop.

Fix sfqdrop() to properly clear q->tail in this situation.

ip netns add lb ip link add dev to-lb type veth peer name in-lb netns lb ethtool -K to-lb tso off # force qdisc to requeue gsoskb ip netns exec lb ethtool -K in-lb gro on # enable NAPI ip link set dev to-lb up ip -netns lb link set dev in-lb up ip addr add dev to-lb 192.168.20.1/24 ip -netns lb addr add dev in-lb 192.168.20.2/24 tc qdisc replace dev to-lb root sfq limit 100

ip netns exec lb netserver

netperf -H 192.168.20.2 -l 100 & netperf -H 192.168.20.2 -l 100 & netperf -H 192.168.20.2 -l 100 & netperf -H 192.168.20.2 -l 100 &