linux-kernel - Re: [PATCH 1/7] netfilter: fix IS_ERR

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Date:	Wed, 17 Feb 2016 09:45:50 +0100
From:	Andrzej Hajda <a.hajda@...sung.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org
Subject: Re: [PATCH 1/7] netfilter: fix IS_ERR_VALUE usage

On 02/17/2016 03:31 AM, Al Viro wrote:
> On Mon, Feb 15, 2016 at 03:35:19PM +0100, Andrzej Hajda wrote:
>> IS_ERR_VALUE should be used only with unsigned long type.
>> Otherwise it can work incorrectly. To achieve this function
>> xt_percpu_counter_alloc is modified to return unsigned long,
>> and its result is assigned to temporary variable to perform
>> error checking, before assigning to .pcnt field.
> 	Wrong fix, IMO.  Just have an anon union of u64 pcnt and
> struct xt_counters __percpu *pcpu in there.  And make this
>
>> +static inline unsigned long xt_percpu_counter_alloc(void)
>>  {
>>  	if (nr_cpu_ids > 1) {
>>  		void __percpu *res = __alloc_percpu(sizeof(struct xt_counters),
>>  						    sizeof(struct xt_counters));
>>  
>>  		if (res == NULL)
>> -			return (u64) -ENOMEM;
>> +			return -ENOMEM;
>>  
>> -		return (u64) (__force unsigned long) res;
>> +		return (__force unsigned long) res;
>>  	}
>>  
>>  	return 0;
> take struct xt_counters * and return 0 or -ENOMEM.  Storing the result of
> allocation in ->pcpu of passed structure.
>
> I mean, look at the callers -
>
>> -	e->counters.pcnt = xt_percpu_counter_alloc();
>> -	if (IS_ERR_VALUE(e->counters.pcnt))
>> +	pcnt = xt_percpu_counter_alloc();
>> +	if (IS_ERR_VALUE(pcnt))
>>  		return -ENOMEM;
>> +	e->counters.pcnt = pcnt;
> should be
> 	if (xt_percpu_counter_alloc(&e->counters) < 0)
> 		return -ENOMEM;
>
> and similar for the rest of callers.  Moreover, if you look at the _users_
> of that fields, you'll see that a bunch of those actually want to use
> ->pcpu instead of doing all those casts.
>
> Really, that's the point - IS_ERR_VALUE is a big red flag saying "we need
> to figure out what's going on in that place", which does include reading
> through the code.  Mechanical "solutions" like that only hide the problem.
>
>
I just tried to make the patch the least invasive :)

The problem with your proposition is that struct xt_counters is exposed
to userspace as well as the structs containing it:
struct arpt_entry,
struct ipt_entry,
struct ip6t_entry

Mixing __percpu pointer into these structures seems problematic.
Maybe it would be better to skip adding union and do ugly casting
in xt_percpu_counter_alloc(struct xt_counters *) and friends.

Regards
Andrzej