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Date:   Thu, 14 Sep 2017 10:35:11 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     nixiaoming <nixiaoming@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, waltje@...lt.nl.mugnet.org,
        gw4pts@...pts.ampr.org, Andrey Konovalov <andreyknvl@...gle.com>,
        Tobias Klauser <tklauser@...tanz.ch>,
        Philip Pettersson <philip.pettersson@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, dede.wu@...wei.com
Subject: Re: [PATCH] net/packet: fix race condition between fanout_add and __unregister_prot_hook

On Thu, Sep 14, 2017 at 10:07 AM, nixiaoming <nixiaoming@...wei.com> wrote:
> From: l00219569 <lisimin@...wei.com>
>
> If fanout_add is preempted after running po-> fanout = match
> and before running __fanout_link,
> it will cause BUG_ON when __unregister_prot_hook call __fanout_unlink
>
> so, we need add mutex_lock(&fanout_mutex) to __unregister_prot_hook

The packet socket code has no shortage of locks, so there are many
ways to avoid the race condition between fanout_add and packet_set_ring.

Another option would be to lock the socket when calling fanout_add:

    -               return fanout_add(sk, val & 0xffff, val >> 16);
    +               lock_sock(sk);
    +               ret = fanout_add(sk, val & 0xffff, val >> 16);
    +               release_sock(sk);
    +               return ret;

But, for consistency, and to be able to continue to make sense of the
locking policy, we should use the most appropriate lock. This
is po->bind_lock, as it ensures atomicity between testing whether
a protocol hook is active through po->running and the actual existence
of that hook on the protocol hook list.

fanout_mutex protects the fanout object's list. Taking that on
__unregister_prot_hook even in the case where fanout is not
used (and __dev_remove_pack is called) complicates locking
in this already complicated code.

> or add spin_lock(&po->bind_lock) before po-> fanout = match
>
> this is a patch for add po->bind_lock in fanout_add
>
> test on linux 4.1.12:
> ./trinity -c setsockopt -C 2 -X &

Thanks for testing!

>
> BUG: failure at net/packet/af_packet.c:1414/__fanout_unlink()!
> Kernel panic - not syncing: BUG!
> CPU: 2 PID: 2271 Comm: trinity-c0 Tainted: G        W  O    4.1.12 #1
> Hardware name: Hisilicon PhosphorHi1382 FPGA (DT)
> Call trace:
> [<ffffffc000209414>] dump_backtrace+0x0/0xf8
> [<ffffffc00020952c>] show_stack+0x20/0x28
> [<ffffffc000635574>] dump_stack+0xac/0xe4
> [<ffffffc000633fb8>] panic+0xf8/0x268
> [<ffffffc0005fa778>] __unregister_prot_hook+0xa0/0x144
> [<ffffffc0005fba48>] packet_set_ring+0x280/0x5b4
> [<ffffffc0005fc33c>] packet_setsockopt+0x320/0x950
> [<ffffffc000554a04>] SyS_setsockopt+0xa4/0xd4
>
> Signed-off-by: nixiaoming <nixiaoming@...wei.com>
> Tested-by: wudesheng <dede.wu@...wei.com>
> ---
>  net/packet/af_packet.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 54a18a8..7a52a3b 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -1446,12 +1446,16 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
>         default:
>                 return -EINVAL;
>         }
> -
> -       if (!po->running)
> +       spin_lock(&po->bind_lock);
> +       if (!po->running) {
> +               spin_unlock(&po->bind_lock);
>                 return -EINVAL;
> +       }
>
> -       if (po->fanout)
> +       if (po->fanout) {
> +               spin_unlock(&po->bind_lock);
>                 return -EALREADY;
> +       }
>
>         mutex_lock(&fanout_mutex);
>         match = NULL;
> @@ -1501,6 +1505,7 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
>         }
>  out:
>         mutex_unlock(&fanout_mutex);
> +       spin_unlock(&po->bind_lock);

This function can call kzalloc with GFP_KERNEL, which may sleep. It is
not correct to sleep while holding a spinlock. Which is why I take the lock
later and test po->running again.

I will clean up that patch and send it for review.

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