net_sched: ets: fix a race in ets_qdisc_change()

Gerrard Tai reported a race condition in ETS, whenever SFQ perturb timer
fires at the wrong time.

The race is as follows:

CPU 0                                 CPU 1
[1]: lock root
[2]: qdisc_tree_flush_backlog()
[3]: unlock root
 |
 |                                    [5]: lock root
 |                                    [6]: rehash
 |                                    [7]: qdisc_tree_reduce_backlog()
 |
[4]: qdisc_put()

This can be abused to underflow a parent's qlen.

Calling qdisc_purge_queue() instead of qdisc_tree_flush_backlog()
should fix the race, because all packets will be purged from the qdisc
before releasing the lock.

Fixes: b05972f01e7d ("net: sched: tbf: don't call qdisc_put() while holding tree lock")
Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Suggested-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250611111515.1983366-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/sch_ets.c b/net/sched/sch_ets.c
index 2c069f0181c62b5387118822dfb7a51fc1b3033a..037f764822b96526329a4715bfcc22c502495f14 100644 (file)
--- a/net/sched/sch_ets.c
+++ b/net/sched/sch_ets.c
@@ -661,7 +661,7 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
        for (i = q->nbands; i < oldbands; i++) {
                if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
                        list_del_init(&q->classes[i].alist);
-               qdisc_tree_flush_backlog(q->classes[i].qdisc);
+               qdisc_purge_queue(q->classes[i].qdisc);
        }
        WRITE_ONCE(q->nstrict, nstrict);
        memcpy(q->prio2band, priomap, sizeof(priomap));