netdev - Re: [PATCH bpf] bpf: sockmap: check update requirements after locking

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Date:   Fri, 7 Feb 2020 22:50:06 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Lorenz Bauer <lmb@...udflare.com>,
        John Fastabend <john.fastabend@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>
Cc:     kernel-team@...udflare.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: sockmap: check update requirements after locking

On 2/7/20 11:37 AM, Lorenz Bauer wrote:
> It's currently possible to insert sockets in unexpected states into
> a sockmap, due to a TOCTTOU when updating the map from a syscall.
> sock_map_update_elem checks that sk->sk_state == TCP_ESTABLISHED,
> locks the socket and then calls sock_map_update_common. At this
> point, the socket may have transitioned into another state, and
> the earlier assumptions don't hold anymore. Crucially, it's
> conceivable (though very unlikely) that a socket has become unhashed.
> This breaks the sockmap's assumption that it will get a callback
> via sk->sk_prot->unhash.
> 
> Fix this by checking the (fixed) sk_type and sk_protocol without the
> lock, followed by a locked check of sk_state.
> 
> Unfortunately it's not possible to push the check down into
> sock_(map|hash)_update_common, since BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB
> run before the socket has transitioned from TCP_SYN_RECV into
> TCP_ESTABLISHED.
> 
> Signed-off-by: Lorenz Bauer <lmb@...udflare.com>
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")

Applied, thanks!