netdev - Re: [PATCH 2/2] Bluetooth: check the buffer size for some messages before parsing

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Date:   Fri, 18 Jan 2019 13:44:54 +0100
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Johan Hedberg <johan.hedberg@...il.com>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Ran Menscher <ran.menscher@...ambasecurity.com>
Subject: Re: [PATCH 2/2] Bluetooth: check the buffer size for some messages
 before parsing

Hi Greg,

>>>> The L2CAP_CONF_EFS and L2CAP_CONF_RFC messages can be sent from
>>>> userspace so their structure sizes need to be checked before parsing
>>>> them.
>>> 
>>> this message is confusing me. How can these be send from userspace?
>> 
>> So claimed the original reporter.  You have the information in your
>> inbox, is it incorrect?
> 
> I am pretty sure he meant that the remote attacker can control it from userspace. This is still a wire protocol and not some socket options.
> 
>>>> 
>>>> Based on a patch from Ran Menscher.
>>>> 
>>>> Reported-by: Ran Menscher <ran.menscher@...ambasecurity.com>
>>>> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>> ---
>>>> net/bluetooth/l2cap_core.c | 12 ++++++++----
>>>> 1 file changed, 8 insertions(+), 4 deletions(-)
>>>> 
>>>> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
>>>> index 93daf94565cf..55e48e6efc2b 100644
>>>> --- a/net/bluetooth/l2cap_core.c
>>>> +++ b/net/bluetooth/l2cap_core.c
>>>> @@ -3361,7 +3361,8 @@ static int l2cap_parse_conf_req(struct l2cap_chan *chan, void *data, size_t data
>>>> 			break;
>>>> 
>>>> 		case L2CAP_CONF_RFC:
>>>> -			if (olen == sizeof(rfc))
>>>> +			if ((olen == sizeof(rfc)) &&
>>>> +			    (endptr - ptr >= L2CAP_CONF_OPT_SIZE + sizeof(rfc)))
>>>> 				memcpy(&rfc, (void *) val, olen);
>>> 
>>> We don’t do ((x == y) && (..)) actually. Using (x == y && ..) is plenty.
>> 
>> Ick, ok, whatever, you all trust that your brains can remember C
>> priority levels, me, I trust ()...
>> 
>> I can fix this up to remove the extra (), but I would like _SOMEONE_ to
>> at least validate that this resolves the reported issues…
> 
> I need to reproduce this and then I can tell you.

so I think that just checking

	if (len < 0)
		break;

will just prevent any of these attacks. Since in theory you can also do this via the options, but then you can leak at max 2 octets.

I posted a simple patch for this. It would be however good if this gets verified that I understood the issues correctly.

Regards

Marcel