ipv6: null-check fib6_node before accessing in __ip6_del_rt_siblings()

syzbot reported a null-ptr-deref in __ip6_del_rt_siblings() [0].

The stack trace hinted towards a null dereference of rt->fib6_node when
fn->leaf is accessed in __ip6_del_rt_siblings(). With
RTNL_FLAG_DOIT_UNLOCKED set, inet6_rtm_delroute() operations run
concurrently without acquiring the RTNL lock. In ip6_route_del(), the
route lookup happens under rcu_read_lock() without acquiring
table->tb6_lock.

Between ip6_route_del() looking up the route and __ip6_del_rt_siblings()
acquiring table->tb6_lock, another thread can modify the routing table.
For example, when an ECMP route is replaced via RTM_NEWROUTE with
NLM_F_REPLACE, fib6_add_rt2node() unlinks all old siblings and sets
iter->fib6_node = NULL. A reproducer was found that triggers this [1].

Add a check to ensure rt->fib6_node is non-null before accessing it.

[0]
KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
RIP: 0010:__ip6_del_rt_siblings+0x31e/0x7c0 net/ipv6/route.c:4056
Call Trace:
 <TASK>
 ip6_route_del+0x1054/0x1110 net/ipv6/route.c:4232
 inet6_rtm_delroute+0x5d7/0x6d0 net/ipv6/route.c:5669
 rtnetlink_rcv_msg+0x802/0xc00 net/core/rtnetlink.c:7132
 netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2556
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x7f5/0x990 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1900
 sock_sendmsg_nosec+0x13a/0x180 net/socket.c:800
 __sock_sendmsg net/socket.c:815 [inline]
 ____sys_sendmsg+0x565/0x870 net/socket.c:2713
 ___sys_sendmsg+0x2a5/0x360 net/socket.c:2767
 __sys_sendmsg net/socket.c:2799 [inline]
 __do_sys_sendmsg net/socket.c:2804 [inline]
 __se_sys_sendmsg net/socket.c:2802 [inline]
 __x64_sys_sendmsg+0x1b7/0x290 net/socket.c:2802
 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
 </TASK>

[1] https://gist.github.com/NamanGulati/0766a1159b6ca61928faaf87425ff899

Fixes: bd11ff421d36 ("ipv6: Get rid of RTNL for SIOCDELRT and RTM_DELROUTE.")
Reported-by: syzbot+a73e5ee0fd534fed75bd@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a9b03f9.04649fcc.10325f.0003.GAE@google.com
Signed-off-by: Naman Gulati <namangulati@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260904180645.706425-1-namangulati@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 9658939511e0..08bd68f1b5bb 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4019,6 +4019,7 @@ static int __ip6_del_rt_siblings(struct fib6_info *rt, struct fib6_config *cfg)
 	struct net *net = info->nl_net;
 	struct sk_buff *skb = NULL;
 	struct fib6_table *table;
+	struct fib6_node *fn;
 	int err = -ENOENT;
 
 	if (rt == net->ipv6.fib6_null_entry)
@@ -4026,9 +4027,13 @@ static int __ip6_del_rt_siblings(struct fib6_info *rt, struct fib6_config *cfg)
 	table = rt->fib6_table;
 	spin_lock_bh(&table->tb6_lock);
 
+	fn = rcu_dereference_protected(rt->fib6_node,
+				       lockdep_is_held(&table->tb6_lock));
+	if (!fn)
+		goto out_unlock;
+
 	if (rt->fib6_nsiblings && cfg->fc_delete_all_nh) {
 		struct fib6_info *sibling, *next_sibling;
-		struct fib6_node *fn;
 
 		/* prefer to send a single notification with all hops */
 		skb = nlmsg_new(rt6_nlmsg_size(rt), GFP_ATOMIC);
@@ -4051,8 +4056,6 @@ static int __ip6_del_rt_siblings(struct fib6_info *rt, struct fib6_config *cfg)
 		 * and emit a replace or delete notification, respectively.
 		 */
 		info->skip_notify_kernel = 1;
-		fn = rcu_dereference_protected(rt->fib6_node,
-					    lockdep_is_held(&table->tb6_lock));
 		if (rcu_access_pointer(fn->leaf) == rt) {
 			struct fib6_info *last_sibling, *replace_rt;