[PATCH v1 net] ipv6: Protect multiple routes insertion by table lock.

[PATCH v1 net] ipv6: Protect multiple routes insertion by table lock.

[Kuniyuki Iwashima]

syzbot reported the warning below. [0]

While call_fib6_multipath_entry_notifiers() is called locklessly,
fib6_siblings could be changed concurrently, leading to the
warning (and later null-ptr-deref).

Let's protect multiple routes insertion with tb6_lock.

[0]:
i != fen6_info->nsiblings
WARNING: drivers/net/netdevsim/fib.c:831 at nsim_fib6_event_init drivers/net/netdevsim/fib.c:831 [inline], CPU#0: syz.0.157/6279
WARNING: drivers/net/netdevsim/fib.c:831 at nsim_fib6_prepare_event drivers/net/netdevsim/fib.c:947 [inline], CPU#0: syz.0.157/6279
WARNING: drivers/net/netdevsim/fib.c:831 at nsim_fib_event_schedule_work drivers/net/netdevsim/fib.c:1003 [inline], CPU#0: syz.0.157/6279
WARNING: drivers/net/netdevsim/fib.c:831 at nsim_fib_event_nb+0x1029/0x11d0 drivers/net/netdevsim/fib.c:1043, CPU#0: syz.0.157/6279
Modules linked in:
CPU: 0 UID: 0 PID: 6279 Comm: syz.0.157 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:nsim_fib6_event_init drivers/net/netdevsim/fib.c:831 [inline]
RIP: 0010:nsim_fib6_prepare_event drivers/net/netdevsim/fib.c:947 [inline]
RIP: 0010:nsim_fib_event_schedule_work drivers/net/netdevsim/fib.c:1003 [inline]
RIP: 0010:nsim_fib_event_nb+0x1029/0x11d0 drivers/net/netdevsim/fib.c:1043
Code: eb 0d 4d 89 fd e8 b7 c5 6c fa be 01 00 00 00 48 89 df e8 5a 38 85 fd 4d 89 ef 4c 8b 6c 24 20 e9 4a f3 ff ff e8 98 c5 6c fa 90 <0f> 0b 90 e9 db fa ff ff 89 e9 80 e1 07 80 c1 03 38 c1 0f 8c df f0
RSP: 0018:ffffc900047bef10 EFLAGS: 00010293
RAX: ffffffff875b59a8 RBX: 0000000000000002 RCX: ffff888079770000
RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000001
RBP: ffffc900047bf0a0 R08: ffff88805c0bb02f R09: 1ffff1100b817605
R10: dffffc0000000000 R11: ffffed100b817606 R12: dffffc0000000000
R13: 0000000000000000 R14: 0000000000000001 R15: ffff88802c792000
FS:  00007f334ba076c0(0000) GS:ffff888123ad0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f334ba06ff8 CR3: 000000007a5b2000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 notifier_call_chain+0x1a5/0x3d0 kernel/notifier.c:85
 atomic_notifier_call_chain+0xda/0x180 kernel/notifier.c:223
 call_fib_notifiers+0x31/0x60 net/core/fib_notifier.c:36
 call_fib6_multipath_entry_notifiers+0xf5/0x160 net/ipv6/ip6_fib.c:428
 ip6_route_multipath_add net/ipv6/route.c:5542 [inline]
 inet6_rtm_newroute+0x14f5/0x1a30 net/ipv6/route.c:5664
 rtnetlink_rcv_msg+0x802/0xc00 net/core/rtnetlink.c:7163
 netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2572
 netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
 netlink_unicast+0x7bd/0x940 net/netlink/af_netlink.c:1361
 netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1916
 sock_sendmsg_nosec+0x14e/0x190 net/socket.c:800
 __sock_sendmsg net/socket.c:818 [inline]
 ____sys_sendmsg+0x54e/0x850 net/socket.c:2715
 ___sys_sendmsg+0x2a5/0x360 net/socket.c:2769
 __sys_sendmmsg+0x273/0x4d0 net/socket.c:2858
 __do_sys_sendmmsg net/socket.c:2885 [inline]
 __se_sys_sendmmsg net/socket.c:2882 [inline]
 __x64_sys_sendmmsg+0xa0/0xc0 net/socket.c:2882
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f334ab9e1d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f334ba07028 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007f334ae26090 RCX: 00007f334ab9e1d9
RDX: 04000000000001f2 RSI: 0000200000000000 RDI: 0000000000000004
RBP: 00007f334ac35888 R08: 0000000000000000 R09: 0000000000000000
R10: 00000000000000fc R11: 0000000000000246 R12: 0000000000000000
R13: 00007f334ae26128 R14: 00007f334ae26090 R15: 00007ffd96688aa8
 </TASK>

Fixes: 169fd62799e8 ("ipv6: Get rid of RTNL for SIOCADDRT and RTM_NEWROUTE.")
Reported-by: syzbot+751b587577d0ebca10f2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6adbaed6.67317f00.38c4bc.001b.GAE@google.com/T/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
syzbot found a way to abuse 1ms sleep in netdevsim and pile up
10000+ routes w/ the delay, so it might report this with this
fix applied:

  unregister_netdevice: waiting for syz_tun to become free. Usage count = 10

I have two patches to fix it, but I think this is net-next
material.

  https://syzkaller.appspot.com/text?tag=Patch&x=15923c21580000
---
 net/ipv6/route.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 94e5ef1c4f83..5926ddbd4349 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -5416,6 +5416,7 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 	struct fib6_info *rt_notif = NULL, *rt_last = NULL;
 	struct nl_info *info = &cfg->fc_nlinfo;
 	struct rt6_nh *nh, *nh_safe;
+	struct fib6_table *table;
 	struct fib6_config r_cfg;
 	struct rtnexthop *rtnh;
 	LIST_HEAD(rt6_nh_list);
@@ -5502,10 +5503,12 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 	 */
 	info->skip_notify_kernel = 1;
 
+	table = rt->fib6_table;
+	spin_lock_bh(&table->tb6_lock);
+
 	err_nh = NULL;
 	list_for_each_entry(nh, &rt6_nh_list, list) {
-		err = __ip6_ins_rt(nh->fib6_info, info, extack);
-
+		err = fib6_add(&table->tb6_root, nh->fib6_info, info, extack);
 		if (err) {
 			if (replace && nhn)
 				NL_SET_ERR_MSG_MOD(extack,
@@ -5560,11 +5563,12 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 
 	/* success ... tell user about new route */
 	ip6_route_mpath_notify(rt_notif, rt_last, info, nlflags);
+	spin_unlock_bh(&table->tb6_lock);
 	goto cleanup;
 
 add_errout:
 	/* send notification for routes that were added so that
-	 * the delete notifications sent by ip6_route_del are
+	 * the delete notifications sent by fib6_del() are
 	 * coherent
 	 */
 	if (rt_notif)
@@ -5574,8 +5578,10 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,
 	list_for_each_entry(nh, &rt6_nh_list, list) {
 		if (err_nh == nh)
 			break;
-		ip6_route_del(&nh->r_cfg, extack);
+		fib6_del(nh->fib6_info, &nh->r_cfg.fc_nlinfo,
+			 RT_DEL_REASON_UNSPEC);
 	}
+	spin_unlock_bh(&table->tb6_lock);
 
 cleanup:
 	list_for_each_entry_safe(nh, nh_safe, &rt6_nh_list, list) {
-- 
2.56.0.307.g5fe7bd680c-goog

[syzbot ci] Re: ipv6: Protect multiple routes insertion by table lock.

[syzbot ci]

syzbot ci has tested the following series

[v1] ipv6: Protect multiple routes insertion by table lock.
https://lore.kernel.org/all/20261023191517.607834-1-kuniyu@google.com
* [PATCH v1 net] ipv6: Protect multiple routes insertion by table lock.

and found the following issue:
WARNING: suspicious RCU usage in rt6_multipath_rebalance

Full report is available here:
https://ci.syzbot.org/series/7d878fa2-f84a-4468-85e2-c2a5e263aeff

***

WARNING: suspicious RCU usage in rt6_multipath_rebalance

tree:      net
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git
base:      7076f684d5132e6a6145e1da2fa74771ecd052e5
arch:      amd64
compiler:  Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
config:    https://ci.syzbot.org/builds/bbb3053f-f23f-4fe3-a9e9-85396428119d/config

IPv6: RTM_NEWROUTE with no NLM_F_CREATE or NLM_F_REPLACE
IPv6: NLM_F_CREATE should be set when creating new route
=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
./include/net/addrconf.h:351 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
locks held by syz.2.3386/18922: 1, last CPU#0:
 #0: ffff888177df3430 (&tb->tb6_lock){+...}-{3:3}, at: inet6_rtm_newroute+0x3dc/0x1ab0

stack backtrace:
CPU: 0 UID: 0 PID: 18922 Comm: syz.2.3386 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150
 lockdep_rcu_suspicious+0x140/0x1d0
 rt6_multipath_rebalance+0xf11/0x1080
 fib6_del+0xada/0x15a0
 inet6_rtm_newroute+0x15b5/0x1ab0
 rtnetlink_rcv_msg+0x802/0xc00
 netlink_rcv_skb+0x226/0x4a0
 netlink_unicast+0x8ef/0xa80
 netlink_sendmsg+0x7f0/0xb20
 sock_sendmsg_nosec+0x14e/0x190
 ____sys_sendmsg+0x54e/0x850
 ___sys_sendmsg+0x2a5/0x360
 __sys_sendmmsg+0x273/0x4e0
 __x64_sys_sendmmsg+0xa0/0xc0
 do_syscall_64+0x166/0x520
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa77459f559
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fa775488028 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007fa774826090 RCX: 00007fa77459f559
RDX: 040000000000009f RSI: 00002000000002c0 RDI: 000000000000000b
RBP: 00007fa774636b20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa774826128 R14: 00007fa774826090 R15: 00007fffd26b46d8
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.

To test a fix for this bug, please reply with `#syz test`
(on a separate line) and attach the patch to the email.

Notes:
- The patch will be applied on top of the tested series (as an
  incremental fix).
- To test a new version of the whole series, please send it directly
  to syzbot@lists.linux.dev.
- Arguments like custom git repos and branches are not supported.

Re: [syzbot ci] Re: ipv6: Protect multiple routes insertion by table lock.

[Kuniyuki Iwashima]

On Sat, 24 Oct 2026 10:18:08 -0700, syzbot ci wrote:
> syzbot ci has tested the following series
> 
> [v1] ipv6: Protect multiple routes insertion by table lock.
> https://lore.kernel.org/all/20261023191517.607834-1-kuniyu@google.com
> * [PATCH v1 net] ipv6: Protect multiple routes insertion by table lock.
> 
> and found the following issue:
> WARNING: suspicious RCU usage in rt6_multipath_rebalance
> 
> Full report is available here:
> https://ci.syzbot.org/series/7d878fa2-f84a-4468-85e2-c2a5e263aeff
> 
> ***
> 
> WARNING: suspicious RCU usage in rt6_multipath_rebalance
> 
> tree:      net
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git
> base:      7076f684d5132e6a6145e1da2fa74771ecd052e5
> arch:      amd64
> compiler:  Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
> config:    https://ci.syzbot.org/builds/bbb3053f-f23f-4fe3-a9e9-85396428119d/config
> 
> IPv6: RTM_NEWROUTE with no NLM_F_CREATE or NLM_F_REPLACE
> IPv6: NLM_F_CREATE should be set when creating new route
> =============================
> WARNING: suspicious RCU usage
> syzkaller #0 Not tainted
> -----------------------------
> ./include/net/addrconf.h:351 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 2, debug_locks = 1
> locks held by syz.2.3386/18922: 1, last CPU#0:
>  #0: ffff888177df3430 (&tb->tb6_lock){+...}-{3:3}, at: inet6_rtm_newroute+0x3dc/0x1ab0
> 
> stack backtrace:
> CPU: 0 UID: 0 PID: 18922 Comm: syz.2.3386 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150
>  lockdep_rcu_suspicious+0x140/0x1d0
>  rt6_multipath_rebalance+0xf11/0x1080
>  fib6_del+0xada/0x15a0

Similar to 8e5f1bb81274 ("ipv6: Narrow down RCU critical section
in inet6_rtm_newroute()."), I had to wrap rt6_multipath_rebalance()
with RCU.

Will squash this:

---8<---
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 0dc6ebee6e56..f7c69c9df411 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -1228,9 +1228,7 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
 			fib6_nsiblings++;
 		}
 		BUG_ON(fib6_nsiblings != rt->fib6_nsiblings);
-		rcu_read_lock();
 		rt6_multipath_rebalance(temp_sibling);
-		rcu_read_unlock();
 	}
 
 	/*
@@ -1274,9 +1272,7 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
 						   sibling->fib6_nsiblings - 1);
 				WRITE_ONCE(rt->fib6_nsiblings, 0);
 				list_del_rcu(&rt->fib6_siblings);
-				rcu_read_lock();
 				rt6_multipath_rebalance(next_sibling);
-				rcu_read_unlock();
 				return err;
 			}
 		}
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 5926ddbd4349..9618e7b635e9 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4875,8 +4875,10 @@ void rt6_multipath_rebalance(struct fib6_info *rt)
 	if (WARN_ON_ONCE(!first))
 		return;
 
+	rcu_read_lock();
 	total = rt6_multipath_total_weight(first);
 	rt6_multipath_upper_bound_set(first, total);
+	rcu_read_unlock();
 }
 
 static int fib6_ifup(struct fib6_info *rt, void *p_arg)
---8<---

Thanks