CVE-2025-39851: vxlan: Fix NPD when refreshing an FDB entry with a nexthop object

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

vxlan: Fix NPD when refreshing an FDB entry with a nexthop object

VXLAN FDB entries can point to either a remote destination or an FDB nexthop group. The latter is usually used in EVPN deployments where learning is disabled.

However, when learning is enabled, an incoming packet might try to refresh an FDB entry that points to an FDB nexthop group and therefore does not have a remote. Such packets should be dropped, but they are only dropped after dereferencing the non-existent remote, resulting in a NPD [1] which can be reproduced using [2].

Fix by dropping such packets earlier. Remove the misleading comment from firstremotercu().

[1] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 13 UID: 0 PID: 361 Comm: mausezahn Not tainted 6.17.0-rc1-virtme-g9f6b606b6b37 #1 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 RIP: 0010:vxlansnoop+0x98/0x1e0 [...] Call Trace: <TASK> vxlanencapbypass+0x209/0x240 encapbypassiflocal+0xb1/0x100 vxlanxmitone+0x1375/0x17e0 vxlanxmit+0x6b4/0x15f0 devhardstartxmit+0x5d/0x1c0 devqueuexmit+0x246/0xfd0 packetsendmsg+0x113a/0x1850 socksendmsg+0x38/0x70 syssendto+0x126/0x180 x64syssendto+0x24/0x30 dosyscall64+0xa4/0x260 entrySYSCALL64afterhwframe+0x4b/0x53

[2] #!/bin/bash

ip address add 192.0.2.1/32 dev lo ip address add 192.0.2.2/32 dev lo

ip nexthop add id 1 via 192.0.2.3 fdb ip nexthop add id 10 group 1 fdb

ip link add name vx0 up type vxlan id 10010 local 192.0.2.1 dstport 12345 localbypass ip link add name vx1 up type vxlan id 10020 local 192.0.2.2 dstport 54321 learning

bridge fdb add 00:11:22:33:44:55 dev vx0 self static dst 192.0.2.2 port 54321 vni 10020 bridge fdb add 00:aa:bb:cc:dd:ee dev vx1 self static nhid 10

mausezahn vx0 -a 00:aa:bb:cc:dd:ee -b 00:11:22:33:44:55 -c 1 -q