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    Subject[PATCH 3.16.y-ckt 037/180] tcp: fix possible deadlock in tcp_send_fin()
    Date
    3.16.7-ckt11 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Eric Dumazet <edumazet@google.com>

    commit d83769a580f1132ac26439f50068a29b02be535e upstream.

    Using sk_stream_alloc_skb() in tcp_send_fin() is dangerous in
    case a huge process is killed by OOM, and tcp_mem[2] is hit.

    To be able to free memory we need to make progress, so this
    patch allows FIN packets to not care about tcp_mem[2], if
    skb allocation succeeded.

    In a follow-up patch, we might abort tcp_send_fin() infinite loop
    in case TIF_MEMDIE is set on this thread, as memory allocator
    did its best getting extra memory already.

    This patch reverts d22e15371811 ("tcp: fix tcp fin memory accounting")

    Fixes: d22e15371811 ("tcp: fix tcp fin memory accounting")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    net/ipv4/tcp_output.c | 20 +++++++++++++++++++-
    1 file changed, 19 insertions(+), 1 deletion(-)

    diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
    index 144af9539ceb..f704fdd6133d 100644
    --- a/net/ipv4/tcp_output.c
    +++ b/net/ipv4/tcp_output.c
    @@ -2675,6 +2675,21 @@ begin_fwd:
    }
    }

    +/* We allow to exceed memory limits for FIN packets to expedite
    + * connection tear down and (memory) recovery.
    + * Otherwise tcp_send_fin() could loop forever.
    + */
    +static void sk_forced_wmem_schedule(struct sock *sk, int size)
    +{
    + int amt, status;
    +
    + if (size <= sk->sk_forward_alloc)
    + return;
    + amt = sk_mem_pages(size);
    + sk->sk_forward_alloc += amt * SK_MEM_QUANTUM;
    + sk_memory_allocated_add(sk, amt, &status);
    +}
    +
    /* Send a fin. The caller locks the socket for us. This cannot be
    * allowed to fail queueing a FIN frame under any circumstances.
    */
    @@ -2697,11 +2712,14 @@ void tcp_send_fin(struct sock *sk)
    } else {
    /* Socket is locked, keep trying until memory is available. */
    for (;;) {
    - skb = sk_stream_alloc_skb(sk, 0, sk->sk_allocation);
    + skb = alloc_skb_fclone(MAX_TCP_HEADER,
    + sk->sk_allocation);
    if (skb)
    break;
    yield();
    }
    + skb_reserve(skb, MAX_TCP_HEADER);
    + sk_forced_wmem_schedule(sk, skb->truesize);
    /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
    tcp_init_nondata_skb(skb, tp->write_seq,
    TCPHDR_ACK | TCPHDR_FIN);

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