netdev - Re: [PATCH net] tun, bpf: fix suspicious RCU usage in tun_{attach,detach}

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Message-ID: <20160331193154.GA63937@ast-mbp.thefacebook.com>
Date:	Thu, 31 Mar 2016 12:31:56 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Miller <davem@...emloft.net>, daniel@...earbox.net,
	eric.dumazet@...il.com, mkubecek@...e.cz, sasha.levin@...cle.com,
	jslaby@...e.cz, mst@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] tun, bpf: fix suspicious RCU usage in
 tun_{attach,detach}_filter

On Thu, Mar 31, 2016 at 09:24:12PM +0200, Hannes Frederic Sowa wrote:
> Hello,
> 
> On 31.03.2016 21:21, David Miller wrote:
> >From: Daniel Borkmann <daniel@...earbox.net>
> >Date: Thu, 31 Mar 2016 14:16:18 +0200
> >
> >>On 03/31/2016 01:59 PM, Eric Dumazet wrote:
> >>>On Thu, 2016-03-31 at 13:35 +0200, Daniel Borkmann wrote:
> >>>
> >>>>+static inline bool sock_owned_externally(const struct sock *sk)
> >>>>+{
> >>>>+	return sk->sk_flags & (1UL << SOCK_EXTERNAL_OWNER);
> >>>>+}
> >>>>+
> >>>
> >>>Have you reinvented sock_flag(sl, SOCK_EXTERNAL_OWNER) ? ;)
> >>>
> >>>Anyway, using a flag for this purpose sounds overkill to me.
> >>
> >>Right.
> >>
> >>>Setting it is a way to 'fool' lockdep anyway...
> >>
> >>Yep, correct, we'd be fooling the tun case, so this diff doesn't
> >>really make it any better there.
> >
> >I like the currently proposed patch where TUN says that RTNL is what
> >the synchronizing element is.
> >
> >Maybe we could make a helper of some sort but since we only have once
> >case like this is just overkill.
> >
> >Alexei, do you really mind if I apply Danile's patch?

I don't have strong opinion either way.
Though Hannes's patch below looks simpler and easier to backport.
Yeah, I do care about backports quite a bit more nowadays :)

> I proposed the following patch to Daniel and he seemed to like it. I
> was just waiting for his feedback and tags and wanted to send it out
> then.
> 
> What do you think?
> 
> lockdep_sock_is_held does also have some other applications in other
> parts of the source.
> 
> Bye,
> Hannes
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 255d3e03727b73..651b84a38cfb9b 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1330,6 +1330,12 @@ static inline void sock_release_ownership(struct sock
> *sk)
>  	sk->sk_lock.owned = 0;
>  }
> 
> +static inline bool lockdep_sock_is_held(struct sock *sk)
> +{
> +	return lockdep_is_held(&sk->sk_lock) ||
> +	       lockdep_is_held(&sk->sk_lock.slock);
> +}
> +
>  /*
>   * Macro so as to not evaluate some arguments when
>   * lockdep is not enabled.
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 4b81b71171b4ce..8ab270d5ce5507 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -1166,7 +1166,8 @@ static int __sk_attach_prog(struct bpf_prog *prog,
> struct sock *sk)
>  	}
> 
>  	old_fp = rcu_dereference_protected(sk->sk_filter,
> -					   sock_owned_by_user(sk));
> +					   lockdep_rtnl_is_held() ||
> +					   lockdep_sock_is_held(sk));
>  	rcu_assign_pointer(sk->sk_filter, fp);
> 
>  	if (old_fp)
> @@ -2259,7 +2260,9 @@ int sk_detach_filter(struct sock *sk)
>  		return -EPERM;
> 
>  	filter = rcu_dereference_protected(sk->sk_filter,
> -					   sock_owned_by_user(sk));
> +					   lockdep_rtnl_is_held() ||
> +					   lockdep_sock_is_held(sk));
> +
>  	if (filter) {
>  		RCU_INIT_POINTER(sk->sk_filter, NULL);
>  		sk_filter_uncharge(sk, filter);
>