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

Affected: Linux

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:vxlan_xmit+0xb58/0x15f0 [...] Call Trace: <TASK> dev_hard_start_xmit+0x5d/0x1c0 __dev_queue_xmit+0x246/0xfd0 packet_sendmsg+0x113a/0x1850 __sock_sendmsg+0x38/0x70 __sys_sendto+0x126/0x180 __x64_sys_sendto+0x24/0x30 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+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…

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