net_sched: sch_sfq: fix a potential crash on gso_skb 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->gso_skb, and an enqueue() on an empty SFQ qdisc can be followed
by an immediate drop.

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

Tested:

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 gso_skb
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 &

Fixes: a53851e ("net: sched: explicit locking in gso_cpu fallback")
Reported-by: Marcus Wichelmann <[email protected]>
Closes: https://lore.kernel.org/netdev/[email protected]/
Signed-off-by: Eric Dumazet <[email protected]>
Reviewed-by: Toke Høiland-Jørgensen <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/sched/sch_sfq.c

@@ -310,7 +310,10 @@ static unsigned int sfq_drop(struct Qdisc *sch, struct sk_buff **to_free)
 		/* It is difficult to believe, but ALL THE SLOTS HAVE LENGTH 1. */
 		x = q->tail->next;
 		slot = &q->slots[x];
-		q->tail->next = slot->next;
+		if (slot->next == x)
+			q->tail = NULL; /* no more active slots */
+		else
+			q->tail->next = slot->next;
 		q->ht[slot->hash] = SFQ_EMPTY_SLOT;
 		goto drop;
 	}