inet: frags: flush pending skbs in fqdir_pre_exit()

We have been seeing occasional deadlocks on pernet_ops_rwsem since
September in NIPA. The stuck task was usually modprobe (often loading
a driver like ipvlan), trying to take the lock as a Writer.
lockdep does not track readers for rwsems so the read wasn't obvious
from the reports.

On closer inspection the Reader holding the lock was conntrack looping
forever in nf_conntrack_cleanup_net_list(). Based on past experience
with occasional NIPA crashes I looked thru the tests which run before
the crash and noticed that the crash follows ip_defrag.sh. An immediate
red flag. Scouring thru (de)fragmentation queues reveals skbs sitting
around, holding conntrack references.

The problem is that since conntrack depends on nf_defrag_ipv6,
nf_defrag_ipv6 will load first. Since nf_defrag_ipv6 loads first its
netns exit hooks run _after_ conntrack's netns exit hook.

Flush all fragment queue SKBs during fqdir_pre_exit() to release
conntrack references before conntrack cleanup runs. Also flush
the queues in timer expiry handlers when they discover fqdir->dead
is set, in case packet sneaks in while we're running the pre_exit
flush.

The commit under Fixes is not exactly the culprit, but I think
previously the timer firing would eventually unblock the spinning
conntrack.

Fixes: d5dd88794a ("inet: fix various use-after-free in defrags units")
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251207010942.1672972-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/inet_frag.h

@@ -123,18 +123,7 @@ void inet_frags_fini(struct inet_frags *);
 
 int fqdir_init(struct fqdir **fqdirp, struct inet_frags *f, struct net *net);
 
-static inline void fqdir_pre_exit(struct fqdir *fqdir)
+void fqdir_pre_exit(struct fqdir *fqdir);
-{
-	/* Prevent creation of new frags.
-	 * Pairs with READ_ONCE() in inet_frag_find().
-	 */
-	WRITE_ONCE(fqdir->high_thresh, 0);
-
-	/* Pairs with READ_ONCE() in inet_frag_kill(), ip_expire()
-	 * and ip6frag_expire_frag_queue().
-	 */
-	WRITE_ONCE(fqdir->dead, true);
-}
 void fqdir_exit(struct fqdir *fqdir);
 
 void inet_frag_kill(struct inet_frag_queue *q, int *refs);

include/net/ipv6_frag.h

@@ -69,9 +69,6 @@ ip6frag_expire_frag_queue(struct net *net, struct frag_queue *fq)
 	int refs = 1;
 
 	rcu_read_lock();
-	/* Paired with the WRITE_ONCE() in fqdir_pre_exit(). */
-	if (READ_ONCE(fq->q.fqdir->dead))
-		goto out_rcu_unlock;
 	spin_lock(&fq->q.lock);
 
 	if (fq->q.flags & INET_FRAG_COMPLETE)
@@ -80,6 +77,12 @@ ip6frag_expire_frag_queue(struct net *net, struct frag_queue *fq)
 	fq->q.flags |= INET_FRAG_DROP;
 	inet_frag_kill(&fq->q, &refs);
 
+	/* Paired with the WRITE_ONCE() in fqdir_pre_exit(). */
+	if (READ_ONCE(fq->q.fqdir->dead)) {
+		inet_frag_queue_flush(&fq->q, 0);
+		goto out;
+	}
+
 	dev = dev_get_by_index_rcu(net, fq->iif);
 	if (!dev)
 		goto out;

net/ipv4/inet_fragment.c

@@ -218,6 +218,41 @@ static int __init inet_frag_wq_init(void)
 
 pure_initcall(inet_frag_wq_init);
 
+void fqdir_pre_exit(struct fqdir *fqdir)
+{
+	struct inet_frag_queue *fq;
+	struct rhashtable_iter hti;
+
+	/* Prevent creation of new frags.
+	 * Pairs with READ_ONCE() in inet_frag_find().
+	 */
+	WRITE_ONCE(fqdir->high_thresh, 0);
+
+	/* Pairs with READ_ONCE() in inet_frag_kill(), ip_expire()
+	 * and ip6frag_expire_frag_queue().
+	 */
+	WRITE_ONCE(fqdir->dead, true);
+
+	rhashtable_walk_enter(&fqdir->rhashtable, &hti);
+	rhashtable_walk_start(&hti);
+
+	while ((fq = rhashtable_walk_next(&hti))) {
+		if (IS_ERR(fq)) {
+			if (PTR_ERR(fq) != -EAGAIN)
+				break;
+			continue;
+		}
+		spin_lock_bh(&fq->lock);
+		if (!(fq->flags & INET_FRAG_COMPLETE))
+			inet_frag_queue_flush(fq, 0);
+		spin_unlock_bh(&fq->lock);
+	}
+
+	rhashtable_walk_stop(&hti);
+	rhashtable_walk_exit(&hti);
+}
+EXPORT_SYMBOL(fqdir_pre_exit);
+
 void fqdir_exit(struct fqdir *fqdir)
 {
 	INIT_WORK(&fqdir->destroy_work, fqdir_work_fn);
@@ -290,6 +325,7 @@ void inet_frag_queue_flush(struct inet_frag_queue *q,
 {
 	unsigned int sum;
 
+	reason = reason ?: SKB_DROP_REASON_FRAG_REASM_TIMEOUT;
 	sum = inet_frag_rbtree_purge(&q->rb_fragments, reason);
 	sub_frag_mem_limit(q->fqdir, sum);
 }

net/ipv4/ip_fragment.c

@@ -134,11 +134,6 @@ static void ip_expire(struct timer_list *t)
 	net = qp->q.fqdir->net;
 
 	rcu_read_lock();
-
-	/* Paired with WRITE_ONCE() in fqdir_pre_exit(). */
-	if (READ_ONCE(qp->q.fqdir->dead))
-		goto out_rcu_unlock;
-
 	spin_lock(&qp->q.lock);
 
 	if (qp->q.flags & INET_FRAG_COMPLETE)
@@ -146,6 +141,13 @@ static void ip_expire(struct timer_list *t)
 
 	qp->q.flags |= INET_FRAG_DROP;
 	inet_frag_kill(&qp->q, &refs);
+
+	/* Paired with WRITE_ONCE() in fqdir_pre_exit(). */
+	if (READ_ONCE(qp->q.fqdir->dead)) {
+		inet_frag_queue_flush(&qp->q, 0);
+		goto out;
+	}
+
 	__IP_INC_STATS(net, IPSTATS_MIB_REASMFAILS);
 	__IP_INC_STATS(net, IPSTATS_MIB_REASMTIMEOUT);