when the number of DRR classes decreases, the round-robin active list can
contain elements that have already been freed in ets_qdisc_change(). As a
consequence, it's possible to see a NULL dereference crash, caused by the
attempt to call cl->qdisc->ops->peek(cl->qdisc) when cl->qdisc is NULL:
v3: fix race between ets_qdisc_change() and ets_qdisc_dequeue() delisting
DRR classes beyond 'nbands' in ets_qdisc_change() with the qdisc lock
acquired, thanks to Cong Wang.
v2: when a NULL qdisc is found in the DRR active list, try to dequeue skb
from the next list item.
Reported-by: Hangbin Liu <liuhangbin@gmail.com> Fixes: dcc68b4d8084 ("net: sch_ets: Add a new Qdisc") Signed-off-by: Davide Caratti <dcaratti@redhat.com> Link: https://lore.kernel.org/r/7a5c496eed2d62241620bdbb83eb03fb9d571c99.1637762721.git.dcaratti@redhat.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>