On 23/11/2021 13:33, Nikolay Aleksandrov wrote:
> On 23/11/2021 13:09, Ido Schimmel wrote:
>> On Tue, Nov 23, 2021 at 12:27:19PM +0200, Nikolay Aleksandrov wrote:
>>> From: Nikolay Aleksandrov <nikolay@xxxxxxxxxx>
>>>
>>> When we try to add an IPv6 nexthop and IPv6 is not enabled
>>> (!CONFIG_IPV6) we'll hit a NULL pointer dereference[1] in the error path
>>> of nh_create_ipv6() due to calling ipv6_stub->fib6_nh_release. The bug
>>> has been present since the beginning of IPv6 nexthop gateway support.
>>> Commit 1aefd3de7bc6 ("ipv6: Add fib6_nh_init and release to stubs") tells
>>> us that only fib6_nh_init has a dummy stub because fib6_nh_release should
>>> not be called if fib6_nh_init returns an error, but the commit below added
>>> a call to ipv6_stub->fib6_nh_release in its error path. To fix it return
>>> the dummy stub's -EAFNOSUPPORT error directly without calling
>>> ipv6_stub->fib6_nh_release in nh_create_ipv6()'s error path.
>>
>> [...]
>>
>>> diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
>>> index a69a9e76f99f..5dbd4b5505eb 100644
>>> --- a/net/ipv4/nexthop.c
>>> +++ b/net/ipv4/nexthop.c
>>> @@ -2565,11 +2565,15 @@ static int nh_create_ipv6(struct net *net, struct nexthop *nh,
>>> /* sets nh_dev if successful */
>>> err = ipv6_stub->fib6_nh_init(net, fib6_nh, &fib6_cfg, GFP_KERNEL,
>>> extack);
>>> - if (err)
>>> + if (err) {
>>> + /* IPv6 is not enabled, don't call fib6_nh_release */
>>> + if (err == -EAFNOSUPPORT)
>>> + goto out;
>>> ipv6_stub->fib6_nh_release(fib6_nh);
>>
>> Is the call actually necessary? If fib6_nh_init() failed, then I believe
>> it should clean up after itself and not rely on fib6_nh_release().
>>
>
> I think it doesn't do that, or at least not entirely. For example take the following
> sequence of events:
> fib6_nh_init:
> ...
> err = fib_nh_common_init(net, &fib6_nh->nh_common, cfg->fc_encap,
> cfg->fc_encap_type, cfg, gfp_flags, extack);
> (passes)
>
> then after:
>
> fib6_nh->rt6i_pcpu = alloc_percpu_gfp(struct rt6_info *, gfp_flags);
> if (!fib6_nh->rt6i_pcpu) {
> err = -ENOMEM;
> goto out;
> }
> (fails)
>
> I don't see anything in the error path that would free the fib_nh_common_init() resources,
> i.e. nothing calls fib_nh_common_release(), which is called by fib6_nh_release().
>
> By the way, I haven't checked but it looks like fib_check_nh_v6_gw() might leak memory if
> fib6_nh_init() fails like that unless I'm missing something.
>
> That change might be doable, but much riskier because there is at least 1 call site which relies
> on fib6_info_release -> fib6_info_destroy_rcu() to call fib6_nh_release in its error path.
>
> I'd prefer to fix these bugs in a straight-forward way and would go with the bigger
> change for fib6_nh_init() cleanup for net-next. WDYT ?
>
> Cheers,
> Nik
>
>
Just to let everyone know, me and Ido had a quick offline discussion about the issue, I'll
try to untangle the places which have different cleanup expectations of fib6_nh_init and
try to make it clean up after itself, as that would fix more bugs (e.g. the memory leak I
mentioned earlier) automatically. If the change is too risky or becomes bigger than expected
we can always continue with the simpler fixes for -net and clean it all up in net-next.
I'll update the thread soon.
Thanks,
Nik
