netdev - Re: Possible fix

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Date:	Sun, 02 Mar 2014 17:26:38 +0100
From:	Nikolay Aleksandrov <nikolay@...hat.com>
To:	Paul Moore <paul@...l-moore.com>,
	Steffen Klassert <steffen.klassert@...unet.com>
CC:	netdev@...r.kernel.org, Dave Jones <davej@...hat.com>,
	Fan Du <fan.du@...driver.com>,
	"David S. Miller" <davem@...emloft.net>,
	linux-security-module@...r.kernel.org
Subject: Re: Possible fix

On 02/28/2014 11:10 PM, Paul Moore wrote:
> On Friday, February 28, 2014 11:10:07 AM Nikolay Aleksandrov wrote:
>> On 02/28/2014 08:23 AM, Steffen Klassert wrote:
>>> On Thu, Feb 27, 2014 at 05:17:37PM +0100, Nikolay Aleksandrov wrote:
>>>> Hi,
>>>> I'm not familiar with the code but happened to see the bug, could you
>>>> try the following patch, I believe it should fix the issue.
>>>>
>>>> Thanks,
>>>>
>>>>  Nik
>>>>
>>>> [PATCH net] net: af_key: fix sleeping under rcu
>>>>
>>>> There's a kmalloc with GFP_KERNEL in a helper
>>>> (pfkey_sadb2xfrm_user_sec_ctx) used in pfkey_compile_policy which is
>>>> called under rcu_read_lock. Adjust pfkey_sadb2xfrm_user_sec_ctx to have
>>>> a gfp argument and adjust the users.
>>>
>>> Looking at the git history, it seems that this bug is about nine
>>> years old. I guess noone is actually using this.
> 
> Most (all?) of the labeled IPsec users use the netlink interface and not pfkey 
> so it isn't surprising that this has gone unnoticed for some time.
> 
>>>> diff --git a/net/key/af_key.c b/net/key/af_key.c
>>>> index 1a04c1329362..1526023f99ed 100644
>>>> --- a/net/key/af_key.c
>>>> +++ b/net/key/af_key.c
>>>> @@ -3239,7 +3240,7 @@ static struct xfrm_policy
>>>> *pfkey_compile_policy(struct sock *sk, int opt,>> 
>>>>  		}
>>>>  		if ((*dir = verify_sec_ctx_len(p)))
>>>>  		
>>>>  			goto out;
>>>>
>>>> -		uctx = pfkey_sadb2xfrm_user_sec_ctx(sec_ctx);
>>>> +		uctx = pfkey_sadb2xfrm_user_sec_ctx(sec_ctx, GFP_ATOMIC);
>>>>
>>>>  		*dir = security_xfrm_policy_alloc(&xp->security, uctx);
>>>
>>> This would fix the allocation done in pfkey_sadb2xfrm_user_sec_ctx().
>>> But security_xfrm_policy_alloc() might call selinux_xfrm_alloc_user()
>>> which does a GFP_KERNEL allocation too. So I guess we also need to fix
>>> selinux.
> 
> Yes, exactly.
> 
>> Right, I just saw that but fixing it at first glance doesn't seem so
>> trivial as we can't pass another argument from compile_policy without
>> changing xfrm_policy_alloc_security's prototype in struct
>> security_operations which AFAICT is doable with some adjustments, but not
>> sure if it's the right thing to do. Changing GFP_KERNEL to GFP_ATOMIC in
>> selinux_xfrm_alloc_user is also a possibility, but there're a many places
>> which use that and can sleep.
> 
> I would recommend adding a gfp_t argument to security_xfrm_policy_alloc() and 
> passing GFP_ATOMIC in pfkey_compile_policy().
> 
Okay, will do.

>> I would extend this patch, but currently don't have the time to search for
>> a nice solution. I can look more into it next week, or if you'd like to
>> take care of it, I wouldn't mind :-)
> 
> It has been this way for a while so I think another day or two isn't going to 
> cause any major harm.  If you are going to put a patch together that's great, 
> CC me and I'll review/ACK it, but if you don't want to bother let me know and 
> I'll work on a patch.
> 
> Thanks,
> -Paul
> 
I'll fix up the patch then and re-submit properly.

Cheers,
 Nik



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