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

[David Ahern <dsahern@kernel.org>]

On 9/21/25 9:08 AM, Ido Schimmel wrote:
> 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@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/68c9a4d2.050a0220.3c6139.0e63.GAE@google.com/
> Tested-by: syzbot+6596516dd2b635ba2350@syzkaller.appspotmail.com
> Signed-off-by: Ido Schimmel <idosch@nvidia.com>
> ---
>  net/ipv4/nexthop.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>