CVE-2025-39851

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 first_remote_rcu().

[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:vxlan_snoop+0x98/0x1e0 [...] Call Trace: <TASK> vxlan_encap_bypass+0x209/0x240 encap_bypass_if_local+0xb1/0x100 vxlan_xmit_one+0x1375/0x17e0 vxlan_xmit+0x6b4/0x15f0 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 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

VXLAN could crash on FDB refresh (learning) when the entry points to an FDB nexthop group rather than a concrete remote, due to dereferencing a NULL remote (first_remote_rcu()), causing a kernel NULL-pointer dereference. The fix drops such packets before accessing the remote and clarifies that a remote is not guaranteed to exist. Impact is a remote DoS in specific VXLAN/EVPN configurations with learning enabled. This issue is hard to trigger purely over the network in normal deployments — it requires a VXLAN setup with learning enabled and FDB entries that point to a nexthop group (nhid) rather than a remote destination. Reproducing it typically needs a local actor with the ability to create/modify network interfaces and bridge FDB entries (e.g. root or a process with network-namespace / admin privileges) to craft the specific nexthop+learning conditions. Remote exploitation is therefore unlikely except in environments where an attacker already has privileged control of the overlay configuration (compromised VM/container or management plane).

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, prevent module vxlan from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loading automatically.