Re: [PATCH 2/2 net v2] net: vxlan: fix nd_tbl NULL dereference when IPv6 is disabled

Fernando Fernandez Mancera
2026-03-02T17:45:00Z

On 3/2/26 5:36 PM, Ido Schimmel wrote:
> On Mon, Mar 02, 2026 at 04:13:14PM +0100, Fernando Fernandez Mancera wrote:
>> On 3/2/26 3:43 PM, Sabrina Dubroca wrote:
>>> 2026-03-02, 15:08:47 +0100, Fernando Fernandez Mancera wrote:
>>>> When booting with the 'ipv6.disable=1' parameter, the nd_tbl is never
>>>> initialized because inet6_init() exits before ndisc_init() is called
>>>> which initializes it. If an IPv6 packet is injected into the interface,
>>>> route_shortcircuit() is called and a NULL pointer dereference happens on
>>>> neigh_lookup().
>>>
>>> VXLAN can be built-in even when IPv6 is built as a module, and then
>>> ipv6_mod_enabled can't be used in VXLAN:
>>>
>>> ld: drivers/net/vxlan/vxlan_core.o: in function `route_shortcircuit':
>>> drivers/net/vxlan/vxlan_core.c:2133:(.text+0x116e0): undefined reference to `ipv6_mod_enabled'
>>>
>>>
>>> GENEVE has a Kconfig tweak to prevent this since 094be0927ff3
>>> ("geneve: fix build with modular IPV6"):
>>>
>>> depends on IPV6 || !IPV6
>>>
>>> but VXLAN doesn't.
>>>
>>
>> Ugh, good catch. Thanks Sabrina.
>>
>>> But then I think we don't need to use ipv6_stub at all.
>>
>> If we enforce "depends on IPV6 || IPV6=n" on VXLAN we could get rid of
>> ipv6_stub completely and use direct calls instead.
>
> I just noticed that this is already enforced for the bridge, so maybe
> you can just remove ipv6_stub in patch #1?
>

Yes, after Sabrina mentioned it that was what I planned to do. But on
VXLAN I want to avoid such patch for net tree as a fix. Instead I
propose to use the __in6_dev_get() check as neigh_reduce() does. Later,
do a follow-up patch on net-next tree removing ipv6_stub and enforcing
"depends on IPV6 || IPV6=n".