netdev - Re: [PATCH 1/1] Revert "af-packet: Use existing netdev reference for bound sockets"

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Date:	Mon, 18 Nov 2013 16:17:02 -0800
From:	Salam Noureddine <noureddine@...stanetworks.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Willem de Bruijn <willemb@...gle.com>,
	Phil Sutter <phil@....cc>, Eric Dumazet <edumazet@...gle.com>,
	netdev@...r.kernel.org, greearb@...delatech.com
Subject: Re: [PATCH 1/1] Revert "af-packet: Use existing netdev reference for
 bound sockets"

Hi Daniel,

Thanks for looking into this. It seems to me that just using a flag
won't help in avoiding the race condition
since packet_notifier can still run after the flag check. We need to
have some sort of locking to avoid it.
One possibility is to grab the po->bind_lock (which is taken in
packet_notifier) in packet_snd for bound
sockets, increment the refcnt for the net_device using dev_hold and
release the lock. Not sure if this is
too expensive though and would defeat the purpose of the original
patch. Another problem could be that it
might delay device unregistering if somebody is sending a lot of
packets on the socket.

Please let me know if I am missing something here and I would be happy
to try the patch if my analysis
is wrong.

Thanks,

Salam

On Mon, Nov 18, 2013 at 3:39 PM, Daniel Borkmann <dborkman@...hat.com> wrote:
> On 11/18/2013 07:03 PM, David Miller wrote:
>>
>> From: Daniel Borkmann <dborkman@...hat.com>
>> Date: Mon, 18 Nov 2013 10:00:53 +0100
>>
>>> On 11/18/2013 08:40 AM, Salam Noureddine wrote:
>>>>
>>>> This reverts commit 827d978037d7d0bf0860481948c6d26ead10042f
>>>> ("af-packet: Use existing netdev reference for bound sockets")
>>>>
>>>> The patch introduced a race condition between packet_snd and
>>>> packet_notifier when a net_device is being unregistered. In the case
>>>> of
>>>> a bound socket, packet_notifier can drop the last reference to the
>>>> net_device and packet_snd might end up sending a packet over a freed
>>>> net_device.
>>>
>>>
>>> So there's no other workaround possible like e.g. setting a flag in
>>> struct packet_sock so that in case our netdevice goes down, we just
>>> set the flag and if set, we return with -ENXIO in send path?
>>> Reverting this would decrease performance for everyone as we would
>>> then do the lookup every time we send a packet again.
>>
>>
>> Agreed, we should try first to find a reasonable fix instead of just
>> doing a knee-jerk revert of this change.
>
>
> Salam, could you give the below a try on your side ?
>
> I tried reproducing this with trafgen, but couldn't so far, meaning
> everything worked (before/after). Having that said, it could also be
> that I'm currently just having a really crappy nic (asix) at hand.
>
> Let me know, thanks !
>
> From 26caa771c0c7d53b28afafc76f1b91ab36a34e73 Mon Sep 17 00:00:00 2001
> From: Daniel Borkmann <dborkman@...hat.com>
> Date: Mon, 18 Nov 2013 23:31:11 +0100
> Subject: [PATCH] packet: don't send packets after we unregistered proto hook
>
> Salam reported a use after free bug in PF_PACKET that occurs when
> we're sending out frames on a socket bound device and suddenly the
> netdevice is being unregistered.
>
> Salam says:
>
>   Commit 827d9780 introduced a race condition between packet_snd
>
>   and packet_notifier when a net_device is being unregistered. In
>   the case of a bound socket, packet_notifier can drop the last
>   reference to the net_device and packet_snd might end up sending
>   a packet over a freed net_device.
>
> To avoid reverting 827d9780, we could make use of po->running member
> that gets reset when we're calling __unregister_prot_hook() in
> packet_notifier() when we receive NETDEV_DOWN notification. In
> that case we receive NETDEV_DOWN before NETDEV_UNREGISTER where
> prot_hook.dev is set to NULL, so that we could make sure to
> leave send path early with -ENETDOWN, which corresponds also
> to our notification.
>
> Fixes: 827d978037d7 ("af-packet: Use existing netdev reference for bound
> sockets.")
> Reported-by: Salam Noureddine <noureddine@...stanetworks.com>
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
> ---
>  net/packet/af_packet.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 2e8286b..e3f64f2 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2094,7 +2094,7 @@ static int tpacket_snd(struct packet_sock *po, struct
> msghdr *msg)
>         reserve = dev->hard_header_len;
>
>         err = -ENETDOWN;
> -       if (unlikely(!(dev->flags & IFF_UP)))
> +       if (unlikely(!(dev->flags & IFF_UP) || !po->running))
>                 goto out_put;
>
>         size_max = po->tx_ring.frame_size
> @@ -2250,7 +2250,7 @@ static int packet_snd(struct socket *sock,
>                 reserve = dev->hard_header_len;
>
>         err = -ENETDOWN;
> -       if (!(dev->flags & IFF_UP))
> +       if (unlikely(!(dev->flags & IFF_UP) || !po->running))
>                 goto out_unlock;
>
>         if (po->has_vnet_hdr) {
> --
> 1.8.3.1
>
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