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

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Message-ID: <2025091904-CVE-2025-39851-8a50@gregkh>
Date: Fri, 19 Sep 2025 17:28:15 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-39851: vxlan: Fix NPD when refreshing an FDB entry with a nexthop object

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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

The Linux kernel CVE team has assigned CVE-2025-39851 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.8 with commit 1274e1cc42264d4e629841e4f182795cb0becfd2 and fixed in 6.12.46 with commit 4ff4f3104da6507e0f118c63c4560dfdeb59dce3
	Issue introduced in 5.8 with commit 1274e1cc42264d4e629841e4f182795cb0becfd2 and fixed in 6.16.6 with commit 0e8630f24c14d9c655d19eabe2e52a9e9f713307
	Issue introduced in 5.8 with commit 1274e1cc42264d4e629841e4f182795cb0becfd2 and fixed in 6.17-rc5 with commit 6ead38147ebb813f08be6ea8ef547a0e4c09559a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-39851
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/net/vxlan/vxlan_core.c
	drivers/net/vxlan/vxlan_private.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4ff4f3104da6507e0f118c63c4560dfdeb59dce3
	https://git.kernel.org/stable/c/0e8630f24c14d9c655d19eabe2e52a9e9f713307
	https://git.kernel.org/stable/c/6ead38147ebb813f08be6ea8ef547a0e4c09559a