linux-kernel - Re: [PATCH] Netfilter: Fix integer overflow in net/ipv6/netfilter/ip6

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Date:	Tue, 23 Mar 2010 12:49:48 +0800
From:	Xiaotian Feng <xtfeng@...il.com>
To:	wzt wzt <wzt.wzt@...il.com>
Cc:	Jan Engelhardt <jengelh@...ozas.de>,
	Patrick McHardy <kaber@...sh.net>,
	linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH] Netfilter: Fix integer overflow in net/ipv6/netfilter/ip6_tables.c

On Tue, Mar 23, 2010 at 11:48 AM, wzt wzt <wzt.wzt@...il.com> wrote:
> 1、 suppose *len = 35,  sizeof(struct ipt_get_entries) = 36
> 2、 set get.size = 0xffffffff from user space
> 3、 sizeof(struct ipt_get_entries) + get.size = 36  + 0xffffffff = 35;
> 4、 if (*len != sizeof(struct ipt_get_entries) + get.size) was bypassed.
>
> you can test with c code:
>
> #include <stdio.h>
>
> int main(void)
> {
>        unsigned int arg = 0xffffffff;
>
>        printf("%u\n", arg + 36);
>        if (35 != arg + 36) {
>                printf("not over flow.\n");
>                return -1;
>        }
>        printf("arg over flow.\n");
>
>        return 0;
> }
>

You didn't get his point... The key point is that sizeof() result type
is size_t, slightly modify you code, try the result.
> int main(void)
> {
>        unsigned int arg = 0xffffffff;
>        unsigned int foo;
>        printf("%lu\n", arg + sizeof(foo));
>        if (sizeof(foo) - 1 != arg + sizeof(foo)) {
>                printf("not over flow.\n");
>                return -1;
>        }
>        printf("arg over flow.\n");
>
>        return 0;
> }

> On Tue, Mar 23, 2010 at 11:04 AM, Jan Engelhardt <jengelh@...ozas.de> wrote:
>>
>> On Tuesday 2010-03-23 03:37, wzt wzt wrote:
>>>> And, for the addition overflow, can it be caught by
>>>>
>>>> "if (*len != sizeof(struct ipt_get_entries) + get.size)"  ???
>>>
>>>sizeof(struct ipt_get_entries) + get.size can be overflow as *len,
>>>get.size is control by user space with copy_from_user().
>>
>> The != should catch it.
>>
>> For 64-bit environments:
>> * + invoked with size_t, unsigned int
>>  => right side promoted to size_t, result type is size_t
>> * != invoked with int and size_t
>>  => left-side promoted to ssize_t (probably; but something as large as size_t)
>> * get.size is 32-bit bounded, as is *len,
>>  so no overflow to worry about at all unless you make
>>  sizeof(X) hilariously big close to 2^64 which is rather unlikely.
>>
>> For 32-bit environments:
>> * Let *len be a number of choice (e.g. 36)
>> * Find a sizeof(X)+get.size that equals 36 mod 2^32.
>> * Since sizeof(X) is const, get.size must be 0 mod 2^32.
>> * So get.size must be a multiple of 2^32 to fool the system.
>> * Since get.size itself is only a 32-bit quantity, you cannot
>>  represent any value larger than 4294967295.
>>
>>
>> What Was What Was Wanted.
>>
>
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