On Tue, Mar 23, 2010 at 11:48 AM, wzt wzt <wzt.wzt@xxxxxxxxx> 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@xxxxxxxxxx> 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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