CVE-2025-39850: vxlan: Fix NPD in {arp,neigh}_reduce() when using nexthop objects

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

vxlan: Fix NPD in {arp,neigh}reduce() when using nexthop objects

When the "proxy" option is enabled on a VXLAN device, the device will suppress ARP requests and IPv6 Neighbor Solicitation messages if it is able to reply on behalf of the remote host. That is, if a matching and valid neighbor entry is configured on the VXLAN device whose MAC address is not behind the "any" remote (0.0.0.0 / ::).

The code currently assumes that the FDB entry for the neighbor's MAC address points to a valid remote destination, but this is incorrect if the entry is associated with an FDB nexthop group. This can result in a NPD [1][3] which can be reproduced using [2][4].

Fix by checking that the remote destination exists before dereferencing it.

[1] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 4 UID: 0 PID: 365 Comm: arping Not tainted 6.17.0-rc2-virtme-g2a89cb21162c #2 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 RIP: 0010:vxlanxmit+0xb58/0x15f0 [...] Call Trace: <TASK> 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 nexthop add id 1 via 192.0.2.2 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 4789 proxy

ip neigh add 192.0.2.3 lladdr 00:11:22:33:44:55 nud perm dev vx0

bridge fdb add 00:11:22:33:44:55 dev vx0 self static nhid 10

arping -b -c 1 -s 192.0.2.1 -I vx0 192.0.2.3

[3] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 13 UID: 0 PID: 372 Comm: ndisc6 Not tainted 6.17.0-rc2-virtmne-g6ee90cb26014 #3 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1v996), BIOS 1.17.0-4.fc41 04/01/2x014 RIP: 0010:vxlanxmit+0x803/0x1600 [...] Call Trace: <TASK> devhardstartxmit+0x5d/0x1c0 devqueuexmit+0x246/0xfd0 ip6finishoutput2+0x210/0x6c0 ip6finishoutput+0x1af/0x2b0 ip6mroutput+0x92/0x3e0 ip6sendskb+0x30/0x90 rawv6sendmsg+0xe6e/0x12e0 socksendmsg+0x38/0x70 syssendto+0x126/0x180 x64syssendto+0x24/0x30 dosyscall64+0xa4/0x260 entrySYSCALL64afterhwframe+0x4b/0x53 RIP: 0033:0x7f383422ec77

[4] #!/bin/bash

ip address add 2001:db8:1::1/128 dev lo

ip nexthop add id 1 via 2001:db8:1::1 fdb ip nexthop add id 10 group 1 fdb

ip link add name vx0 up type vxlan id 10010 local 2001:db8:1::1 dstport 4789 proxy

ip neigh add 2001:db8:1::3 lladdr 00:11:22:33:44:55 nud perm dev vx0

bridge fdb add 00:11:22:33:44:55 dev vx0 self static nhid 10

ndisc6 -r 1 -s 2001:db8:1::1 -w 1 2001:db8:1::3 vx0