netdev - Re: [PATCH net] l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap

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Date:   Wed, 24 Apr 2019 11:29:25 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        James Chapman <jchapman@...alix.com>
Subject: Re: [PATCH net] l2tp: use rcu_dereference_sk_user_data() in l2tp_udp_encap_recv()

On Wed, Apr 24, 2019 at 11:21 AM Guillaume Nault <gnault@...hat.com> wrote:

> It seems that callers of rcu_assign_sk_user_data() can indeed sleep.
>
> But I think we have more problems. Not all code paths treat
> ->sk_user_data as RCU pointers (IIUC that's why we created the
> __sk_user_data() macro, instead of just redefining ->sk_user_data as
> "void __rcu *"). But even if RCU rules were respected for all accesses,
> we'd need to ensure consistent protection for the update side.

Sure.

>
> And then, we'd need to make sure that ->sk_user_data is in sync with
> the encap_rcv() callback (or whatever actually uses the data pointed
> to). Otherwise a module could treat ->sk_user_data as a struct foo
> pointer while it actually points to a struct bar.
>
> For example, a quick look at net/sunrpc/svcsock.c seems to indicate
> that svc_addsock() would accept any (unconnected) UDP socket and pass
> it to svc_addsock(), which in turn would override ->sk_user_data with
> a struct svc_sock pointer. If the socket was previously set up by L2TP,
> then we'd end up with ->sk_user_data pointing to a svc_sock structure,
> but ->encap_rcv still pointing to l2tp_udp_encap_recv(). That's going
> to give unexpected results when l2tp_udp_encap_recv() will dereference
> ->sk_user_data to access (what it believes to be) its tunnel structure.

A full audit is needed, and I have started it. If you want to help
just send a patch ;)

I have looked at this l2tp code only after fixing another issue in
RXRPC, and would have
looked later at SUNRPC.