ipv4: fib_rule: Move fib4_rules_exit() to ->exit().

syzbot reported use-after-free of net->ipv4.rules_ops. [0]

It can be reproduced with these commands:

  while true; do
  	ip netns add ns1
  	ip -n ns1 link set dev lo up
  	ip -n ns1 address add 192.0.2.1/24 dev lo
  	ip -n ns1 link add name dummy1 up type dummy
  	ip -n ns1 address add 198.51.100.1/24 dev dummy1
  	ip -n ns1 rule add ipproto tcp sport 12345 table 12345
  	ip -n ns1 fou add port 5555 ipproto 47 local 192.0.2.1 peer 198.51.100.2 peer_port 54321
  	ip netns del ns1
  done

The cited commit moved fib4_rules_exit() earlier to ->exit_rtnl(),
but the kernel socket destroyed in ->exit() could eventually reach
__fib_lookup().

I left fib4_rules_exit() in ->exit_rtnl() because fib4_rule_delete()
calls fib_unmerge(), which requires RTNL.

However, when ->delete() is called, ->configure() has already been
called, thus fib_unmerge() in ->delete() has no effect.

Let's remove fib_unmerge() in fib4_rule_delete() and move
fib4_rules_exit() to ->exit().

Many thanks to Ido Schimmel for providing the nice repro very quickly.

Note that we can make fib_rules_ops.delete() return void once
net-next opens.

[0]:
BUG: KASAN: slab-use-after-free in fib_rules_lookup+0x15e/0xeb0 net/core/fib_rules.c:321
Read of size 8 at addr ffff88804ec4c680 by task kworker/u8:21/12641

CPU: 0 UID: 0 PID: 12641 Comm: kworker/u8:21 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Workqueue: netns cleanup_net
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_address_description+0x55/0x1e0 mm/kasan/report.c:378
 print_report+0x58/0x70 mm/kasan/report.c:482
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 fib_rules_lookup+0x15e/0xeb0 net/core/fib_rules.c:321
 __fib_lookup+0x106/0x210 net/ipv4/fib_rules.c:96
 ip_route_output_key_hash_rcu+0x294/0x2720 net/ipv4/route.c:2811
 ip_route_output_key_hash+0x18d/0x2a0 net/ipv4/route.c:2702
 __ip_route_output_key include/net/route.h:169 [inline]
 ip_route_output_flow+0x2a/0x150 net/ipv4/route.c:2929
 ip4_datagram_release_cb+0x89d/0xbe0 net/ipv4/datagram.c:118
 release_sock+0x206/0x260 net/core/sock.c:3861
 inet_shutdown+0x2b1/0x390 net/ipv4/af_inet.c:950
 udp_tunnel_sock_release+0x6d/0x80 net/ipv4/udp_tunnel_core.c:197
 fou_release net/ipv4/fou_core.c:562 [inline]
 fou_exit_net+0x17d/0x1f0 net/ipv4/fou_core.c:1230
 ops_exit_list net/core/net_namespace.c:199 [inline]
 ops_undo_list+0x43d/0x8d0 net/core/net_namespace.c:252
 cleanup_net+0x572/0x810 net/core/net_namespace.c:702
 process_one_work kernel/workqueue.c:3314 [inline]
 process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3397
 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3478
 kthread+0x389/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

Fixes: 759923cf03b0 ("ipv4: fib: Convert fib_net_exit_batch() to ->exit_rtnl().")
Reported-by: syzbot+965506b59a2de0b6905c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6a315824.b0403584.28d0ff.0000.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260616191359.4142661-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 8 insertions, 13 deletions

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index c7d1f31650d7..42212970d735 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1612,10 +1612,6 @@ static void ip_fib_net_exit(struct net *net)
 			fib_free_table(tb);
 		}
 	}
-
-#ifdef CONFIG_IP_MULTIPLE_TABLES
-	fib4_rules_exit(net);
-#endif
 }
 
 static int __net_init fib_net_init(struct net *net)
@@ -1652,6 +1648,9 @@ out_semantics:
 	ip_fib_net_exit(net);
 	rtnl_net_unlock(net);
 
+#ifdef CONFIG_IP_MULTIPLE_TABLES
+	fib4_rules_exit(net);
+#endif
 	kfree(net->ipv4.fib_table_hash);
 	fib4_notifier_exit(net);
 	goto out;
@@ -1671,6 +1670,9 @@ static void __net_exit fib_net_exit_rtnl(struct net *net,
 
 static void __net_exit fib_net_exit(struct net *net)
 {
+#ifdef CONFIG_IP_MULTIPLE_TABLES
+	fib4_rules_exit(net);
+#endif
 	kfree(net->ipv4.fib_table_hash);
 	fib4_notifier_exit(net);
 	fib4_semantics_exit(net);
diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
index 51f0193092f0..e068a5bace73 100644
--- a/net/ipv4/fib_rules.c
+++ b/net/ipv4/fib_rules.c
@@ -352,24 +352,17 @@ errout:
 static int fib4_rule_delete(struct fib_rule *rule)
 {
 	struct net *net = rule->fr_net;
-	int err;
-
-	/* split local/main if they are not already split */
-	err = fib_unmerge(net);
-	if (err)
-		goto errout;
 
 #ifdef CONFIG_IP_ROUTE_CLASSID
 	if (((struct fib4_rule *)rule)->tclassid)
 		atomic_dec(&net->ipv4.fib_num_tclassid_users);
 #endif
-	net->ipv4.fib_has_custom_rules = true;
 
 	if (net->ipv4.fib_rules_require_fldissect &&
 	    fib_rule_requires_fldissect(rule))
 		net->ipv4.fib_rules_require_fldissect--;
-errout:
-	return err;
+
+	return 0;
 }
 
 static int fib4_rule_compare(struct fib_rule *rule, struct fib_rule_hdr *frh,