netdev - Re: [PATCH net] tcp: fix tcp_set_congestion

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Message-ID: <2B90C3B0-0A52-4D0A-ADA7-63AAC3C4414D@fb.com>
Date:   Fri, 19 Jul 2019 02:55:44 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
CC:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net] tcp: fix tcp_set_congestion_control() use from bpf
 hook

On 7/18/19, 7:28 PM, "Eric Dumazet" <edumazet@...gle.com> wrote:

    Neal reported incorrect use of ns_capable() from bpf hook.
    
    bpf_setsockopt(...TCP_CONGESTION...)
      -> tcp_set_congestion_control()
       -> ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)
        -> ns_capable_common()
         -> current_cred()
          -> rcu_dereference_protected(current->cred, 1)
    
    Accessing 'current' in bpf context makes no sense, since packets
    are processed from softirq context.
    
    As Neal stated : The capability check in tcp_set_congestion_control()
    was written assuming a system call context, and then was reused from
    a BPF call site.
    
    The fix is to add a new parameter to tcp_set_congestion_control(),
    so that the ns_capable() call is only performed under the right
    context.
    
    Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
    Signed-off-by: Eric Dumazet <edumazet@...gle.com>
    Cc: Lawrence Brakmo <brakmo@...com>
    Reported-by: Neal Cardwell <ncardwell@...gle.com>
    ---

Acked-by: Lawrence Brakmo <brakmo@...com>
Thanks, Eric!