netdev - Re: [RFC PATCH] xfrm: fix handling of XFRM policies mark and mask.

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Message-Id: <D4563759-CEC5-4F33-BF0A-9FA10716F8DF@telecom-bretagne.eu>
Date:	Wed, 6 Feb 2013 14:53:48 +0100
From:	Emmanuel Thierry <emmanuel.thierry@...ecom-bretagne.eu>
To:	jamal <j.hadi123@...il.com>
Cc:	Steffen Klassert <steffen.klassert@...unet.com>,
	Romain KUNTZ <r.kuntz@...lavors.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	herbert@...dor.apana.org.au,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jamal Hadi Salim <hadi@...erus.ca>
Subject: Re: [RFC PATCH] xfrm: fix handling of XFRM policies mark and mask.


Hello,

Le 6 févr. 2013 à 14:14, jamal a écrit :

> 
> On 13-02-05 03:12 AM, Steffen Klassert wrote:
>>> For example, executing the below commands in that order succeed:
>>>  ip -6 xfrm policy flush
>>>  ip -6 xfrm policy add src fd00::1/128 dst fd00::2/128 dir out mark 1 mask 0xffffffff
>>>  ip -6 xfrm policy add src fd00::1/128 dst fd00::2/128 dir out
>> The policy with mark 1 is the first we find. The policy passes the
>> mark check and if the flow matches the selectors, we use this policy.
>> 
>>> But it fails in the reverse order:
>>>  ip -6 xfrm policy flush
>>>  ip -6 xfrm policy add src fd00::1/128 dst fd00::2/128 dir out
>>>  ip -6 xfrm policy add src fd00::1/128 dst fd00::2/128 dir out mark 1 mask 0xffffffff
>>>  RTNETLINK answers: File exists
>> With this scenario, we would find the policy with mark and mask 0 first.
>> This policy passes the mark check too. So we would use this policy if the
>> flow matches the selectors, but the flow asked for a policy with mark 1.
> 
> I think the intent Romain is expressing is reasonable and should resolved at
> insertion time(xfrm_policy_insert()).
> i.e even though the policy (such as mark=1) is inserted afterwards, at
> insertion time if it proves it is more specific and not duplicate, it should be
> inserted ahead of the mark=0.
> The runtime check will work then.

Actually, we didn't think about this problem since we work with priorities, putting the default policy (without a mark) at a minor priority than the marked one.
Your remark makes clearer the ideas behind the design of XFRM, but this leads to an interesting concern. If on policy insertion, the policy were inserted depending on the accuracy of the mark (the more the mask is specific, the more the mark must be put at the beginning of the list), how would we decide which is the more specific between these ones ?

ip -6 xfrm policy add src fd00::1/128 dst fd00::2/128 dir out mark 0x00000001 mask 0x00000005

ip -6 xfrm policy add src fd00::1/128 dst fd00::2/128 dir out mark 0x00000001 mask 0x00000003

Best regards
Emmanuel Thierry

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