CVE-2025-39980: nexthop: Forbid FDB status change while nexthop is in a group

In the Linux kernel, the following vulnerability has been resolved:

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:fiblookupgoodnhc+0x1e/0x80 [...] Call Trace: <TASK> fibtablelookup+0x541/0x650 iprouteoutputkeyhashrcu+0x2ea/0x970 iprouteoutputkeyhash+0x55/0x80 ip4datagramconnect+0x250/0x330 udpconnect+0x2b/0x60 sysconnect+0x9c/0xd0 x64sysconnect+0x18/0x20 dosyscall64+0xa4/0x2a0 entrySYSCALL64afterhwframe+0x4b/0x53