netdev - Re: [PATCH 1/7] xfrm: remove policy lock when accessing policy->walk.dead

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Date:	Tue, 30 Mar 2010 17:01:47 +0300
From:	Timo Teräs <timo.teras@....fi>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	netdev@...r.kernel.org
Subject: Re: [PATCH 1/7] xfrm: remove policy lock when accessing policy->walk.dead

Timo Teräs wrote:
> Herbert Xu wrote:
>> On Tue, Mar 30, 2010 at 07:55:07AM +0300, Timo Teräs wrote:
>>> Herbert Xu wrote:
>>>> On Mon, Mar 29, 2010 at 05:12:38PM +0300, Timo Teras wrote:
>>>>> @@ -1132,7 +1119,7 @@ int xfrm_sk_policy_insert(struct sock *sk, 
>>>>> int dir, struct xfrm_policy *pol)
>>>>>          __xfrm_policy_link(pol, XFRM_POLICY_MAX+dir);
>>>>>      }
>>>>>      if (old_pol)
>>>>> -        __xfrm_policy_unlink(old_pol, XFRM_POLICY_MAX+dir);
>>>>> +        old_pol = __xfrm_policy_unlink(old_pol, XFRM_POLICY_MAX+dir);
>>>>>      write_unlock_bh(&xfrm_policy_lock);
>>>>>       if (old_pol) {
>>>> So when can this actually fail?
>>> Considering that the socket reference is received from the 
>>> sk->sk_policy,
>>> and the hash bucket we use is "XFRM_POLICY_MAX+dir", it's non-obvious if
>>> it can fail or not.
>>>
>>> It would look like the timer can kill a policy and unlink it, but it
>>> would still be found from sk_policy.
>>
>> Socket policies cannot expire.
> 
> Was not aware of that. The above is not needed then.

Since the exported function xfrm_policy_byid() can result in deletion
of socket policy, it's safer to leave this change in. This is can be
even triggered via xfrm_user since it does not check 'dir' for the
policy expired message it handles. Any custom module could do similar
harm.
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