netdev - [PATCH net 1/3] nexthop: Forbid FDB status change while nexthop is in a group

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Message-ID: <20250921150824.149157-2-idosch@nvidia.com>
Date: Sun, 21 Sep 2025 18:08:22 +0300
From: Ido Schimmel <idosch@...dia.com>
To: <netdev@...r.kernel.org>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<edumazet@...gle.com>, <dsahern@...nel.org>, <horms@...nel.org>,
	<petrm@...dia.com>, <aroulin@...dia.com>, Ido Schimmel <idosch@...dia.com>
Subject: [PATCH net 1/3] nexthop: Forbid FDB status change while nexthop is in a group

The kernel forbids the creation of non-FDB nexthop groups with FDB
nexthops:

 # ip nexthop add id 1 via 192.0.2.1 fdb
 # ip nexthop add id 2 group 1
 Error: Non FDB nexthop group cannot have fdb nexthops.

And vice versa:

 # ip nexthop add id 3 via 192.0.2.2 dev dummy1
 # ip nexthop add id 4 group 3 fdb
 Error: FDB nexthop group can only have fdb nexthops.

However, as long as no routes are pointing to a non-FDB nexthop group,
the kernel allows changing the type of a nexthop from FDB to non-FDB and
vice versa:

 # ip nexthop add id 5 via 192.0.2.2 dev dummy1
 # ip nexthop add id 6 group 5
 # ip nexthop replace id 5 via 192.0.2.2 fdb
 # echo $?
 0

This configuration is invalid and can result in a NPD [1] since FDB
nexthops are not associated with a nexthop device:

 # ip route add 198.51.100.1/32 nhid 6
 # ping 198.51.100.1

Fix by preventing nexthop FDB status change while the nexthop is in a
group:

 # ip nexthop add id 7 via 192.0.2.2 dev dummy1
 # ip nexthop add id 8 group 7
 # ip nexthop replace id 7 via 192.0.2.2 fdb
 Error: Cannot change nexthop FDB status while in a group.

[1]
BUG: kernel NULL pointer dereference, address: 00000000000003c0
[...]
Oops: Oops: 0000 [#1] SMP
CPU: 6 UID: 0 PID: 367 Comm: ping Not tainted 6.17.0-rc6-virtme-gb65678cacc03 #1 PREEMPT(voluntary)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014
RIP: 0010:fib_lookup_good_nhc+0x1e/0x80
[...]
Call Trace:
 <TASK>
 fib_table_lookup+0x541/0x650
 ip_route_output_key_hash_rcu+0x2ea/0x970
 ip_route_output_key_hash+0x55/0x80
 __ip4_datagram_connect+0x250/0x330
 udp_connect+0x2b/0x60
 __sys_connect+0x9c/0xd0
 __x64_sys_connect+0x18/0x20
 do_syscall_64+0xa4/0x2a0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Fixes: 38428d68719c ("nexthop: support for fdb ecmp nexthops")
Reported-by: syzbot+6596516dd2b635ba2350@...kaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/68c9a4d2.050a0220.3c6139.0e63.GAE@google.com/
Tested-by: syzbot+6596516dd2b635ba2350@...kaller.appspotmail.com
Signed-off-by: Ido Schimmel <idosch@...dia.com>
---
 net/ipv4/nexthop.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 29118c43ebf5..34137768e7f9 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -2399,6 +2399,13 @@ static int replace_nexthop_single(struct net *net, struct nexthop *old,
 		return -EINVAL;
 	}
 
+	if (!list_empty(&old->grp_list) &&
+	    rtnl_dereference(new->nh_info)->fdb_nh !=
+	    rtnl_dereference(old->nh_info)->fdb_nh) {
+		NL_SET_ERR_MSG(extack, "Cannot change nexthop FDB status while in a group");
+		return -EINVAL;
+	}
+
 	err = call_nexthop_notifiers(net, NEXTHOP_EVENT_REPLACE, new, extack);
 	if (err)
 		return err;
-- 
2.51.0