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

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Date:	Tue, 23 Mar 2010 13:49:38 +0800
From:	wzt wzt <wzt.wzt@...il.com>
To:	Xiaotian Feng <xtfeng@...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

> 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;
>> }

sizeof() is size_t(unsigned int), get.size is also unsigned int,  arg
+ sizeof(foo) can overflow as sizeof(foo) - 1
[root@...alhost test]# ./test
3
arg over flow.

my fault is *len is check with:
        if (*len < sizeof(get)) {
                duprintf("get_entries: %u < %zu\n", *len, sizeof(get));
                return -EINVAL;
        }

so, *len must >= sizeof(get), then:

        if (*len != sizeof(struct ipt_get_entries) + get.size) {
                duprintf("get_entries: %u != %zu\n",
                         *len, sizeof(get) + get.size);
                return -EINVAL;
        }
if sizeof(struct ipt_get_entries) + get.size oveflow, it must <
sizeof(struct ipt_get_entries),  It not make any problem, just return.
get.size overflow can't make any problem,  thx Feng and Jan for helping me.


On Tue, Mar 23, 2010 at 12:49 PM, Xiaotian Feng <xtfeng@...il.com> wrote:
> 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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