syzkaller: NULL dereference in `__mptcp_propagate_sndbuf()`

[mjmartineau]

Eric Dumazet reports that syzbot found a possible NULL dereference:

> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index df208666fd19..2b43577f952e 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -421,6 +421,7 @@ static bool subflow_use_different_dport(struct mptcp_sock *msk, const struct soc
>
>  void __mptcp_set_connected(struct sock *sk)
>  {
> +       __mptcp_propagate_sndbuf(sk, mptcp_sk(sk)->first);

->first can be NULL here, according to syzbot.

https://lore.kernel.org/netdev/CANn89iLZUA6S2a=K8GObnS62KK6Jt4B7PsAs7meMFooM8xaTgw@mail.gmail.com/

[mjmartineau]

More details here:

https://syzkaller.appspot.com/bug?extid=9dfbaedb6e6baca57a32

Patch:

https://patchwork.kernel.org/project/mptcp/patch/3350eaf14a073538bf491f93fc852cd02ab0875a.1699280113.git.pabeni@redhat.com/