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From: Willem de Bruijn <willemb@google.com>
Date: Tue, 26 Jul 2016 13:17:36 -0400
Message-ID: <CA+FuTSdzHMrhcYHMafnrEs=BqzohT36maH6L=yhOppp_tQHQtw@mail.gmail.com>
Subject: Re: [PATCH v2] bluetooth, bpf: split sk_filter in l2cap_sock_recv_cb
To: Mat Martineau <mathew.j.martineau@linux.intel.com>
Cc: linux-bluetooth@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
	Marcel Holtmann <marcel@holtmann.org>, Gustavo Padovan <gustavo.padovan@collabora.co.uk>,
	Alexei Starovoitov <ast@kernel.org>
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List-ID: <linux-bluetooth.vger.kernel.org>

On Tue, Jul 26, 2016 at 1:00 PM, Mat Martineau
<mathew.j.martineau@linux.intel.com> wrote:
>
> On Tue, 26 Jul 2016, Willem de Bruijn wrote:
>
>>> I modified the original patch to call sk_filter for ERTM before the
>>> packet is
>>> handled by the state machine and to not set the filter locked flag. I
>>> tested
>>> using l2test in ERTM mode, with and without a "randomly drop 1 in 64
>>> packets"
>>> filter attached.
>>
>>
>> Thanks for testing. For consistency's sake, is it preferable to filter
>> at this point for all modes?
>
>
> Only ERTM and streaming mode end up on this code path, and I think there's a
> benefit to handling these two modes similarly. There are a number of other
> paths to l2cap_sock_recv_cb(), and there isn't one perfect place to call
> sk_filter for all modes.

Okay.
>
>>
>>>
>>> Mat
>>>
>>> ---
>>>  net/bluetooth/l2cap_core.c |  4 ++++
>>>  net/bluetooth/l2cap_sock.c | 13 +++++++++++--
>>>  2 files changed, 15 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
>>> index 54ceb1f..d5de0ce 100644
>>> --- a/net/bluetooth/l2cap_core.c
>>> +++ b/net/bluetooth/l2cap_core.c
>>> @@ -32,6 +32,7 @@
>>>
>>>  #include <linux/debugfs.h>
>>>  #include <linux/crc16.h>
>>> +#include <linux/filter.h>
>>>
>>>  #include <net/bluetooth/bluetooth.h>
>>>  #include <net/bluetooth/hci_core.h>
>>> @@ -6610,6 +6611,9 @@ static int l2cap_data_rcv(struct l2cap_chan *chan,
>>> struct sk_buff *skb)
>>>                 goto drop;
>>>         }
>>>
>>> +       if (chan->mode == L2CAP_MODE_ERTM && sk_filter(chan->data, skb))
>>> +               goto drop;
>>> +
>>
>>
>> sk_filter can also accept, but trim, packets. If the protocol expects
>> a header that it unconditionally pulls later, use sk_filter_trim to
>
>
> sk_filter_trim_cap? I see that you added that recently. It's not in
> bluetooth-next and we're aiming for a patch that can be easily backported to
> stable.
>
>> avoid trimming to below header length. I think I saw a path from
>>
>>  l2cap_data_rcv
>>    l2cap_rx
>>      l2cap_rx_state_recv
>>        l2cap_reassemble_sdu
>>          case L2CAP_SAR_START
>>            skb_pull(skb, L2CAP_SDULEN_SIZE)
>
>
> How about checking skb->len before that skb_pull instead?

That works, preferably using pskb_may_pull. The only downside of doing
that exactly in this path, is that it is not easy to verify that all
other code paths downstream from sk_filter are also safe. If all
expect this header, the check is best added right after filter.

>
> --
> Mat Martineau
> Intel OTC