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net/sched: sch_qfq: do not free existing class in qfq_change_class()

Fixes qfq_change_class() error case.

cl->qdisc and cl should only be freed if a new class and qdisc
were allocated, or we risk various UAF.

Fixes: 462dbc9101ac ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
Reported-by: syzbot+07f3f38f723c335f106d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6965351d.050a0220.eaf7.00c5.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260112175656.17605-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

authored by

Eric Dumazet and committed by
Jakub Kicinski 3879cffd dbe6b313

+4 -2
+4 -2
net/sched/sch_qfq.c
··· 529 529 return 0; 530 530 531 531 destroy_class: 532 - qdisc_put(cl->qdisc); 533 - kfree(cl); 532 + if (!existing) { 533 + qdisc_put(cl->qdisc); 534 + kfree(cl); 535 + } 534 536 return err; 535 537 } 536 538