linux-kernel - Re: [PATCH 2/2] tracing/filters: use strcmp() instead of strncmp()

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Message-ID: <4A247875.9010507@cn.fujitsu.com>
Date:	Tue, 02 Jun 2009 08:55:17 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Tom Zanussi <tzanussi@...il.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tracing/filters: use strcmp() instead of strncmp()

Frederic Weisbecker wrote:
> On Mon, Jun 01, 2009 at 01:45:47PM +0800, Li Zefan wrote:
>>>>>> I don't think there's any security issue. It's irrelevant how big the user-input
>>>>>> strings are. The point is those strings are guaranteed to be NULL-terminated.
>>>>>> Am I missing something?
>>>>>>
>>>>>> And I don't think it's necessary to make 2 patches that each patch converts
>>>>>> one strncmp to strcmp. But maybe it's better to improve this changelog?
>>>>> Hmm, you must be right, indeed they seem to be guaranted beeing NULL-terminated
>>>>> strings.
>>>>>
>>>> Sorry, I was wrong. :(
>>>>
>>>> Though the user-input strings are guaranted to be NULL-terminated, strings
>>>> generated by TRACE_EVENT might not.
>>>>
>>>> We define static strings this way:
>>>> 	TP_struct(
>>>> 		__array(char, foo, LEN)
>>>> 	)
>>>> But foo is not necessarily a string, though I doubt someone will use it
>>>> as non-string char array.
>>>
>>> Yeah, but the user defined comparison operand is NULL terminated.
>>> So the strcmp will stop at this boundary.
>>>
>> The user input string is NULL terminated and is limited to MAX_FILTER_STR_VAL,
>> and it's strcmp() not strcpy(), but it's still unsafe. No?
>>
>> 	cmp = strcmp(addr, pred->str_val);
>>
>> If addr is not NULL-terminated string but char array, and length of
>> str_val > length of addr, then we'll be exceeding the boundary of the
>> array.
> 
> 
> 
> No, once both strings appear to be different, strcmp returns.
> As an example, the generic strcmp in lib/string.c is as follows:
> 
> int strcmp(const char *cs, const char *ct)
> {
> 	signed char __res;
> 
> 	while (1) {
> 		if ((__res = *cs - *ct++) != 0 || !*cs++)
> 			break;
> 	}
> 	return __res;
> }
> 
> Once cs[n] != ct[n], or !cs[n] || !ct[n], strcmp() stops,
> and the x86 implementation does exactly the same.
> 
> So I guess it's safe.
> 

See this example:

cmp = strcmp(addr, pred->str_val);

length(addr) == 6, strlen(str_val) == 10

         123456
addr:    abcdef?
               ^
               |
               v
str_val: abcdefzzzz\0

or the 2 happen to match even after addr overflowed:

         123456
addr:    abcdefzzzz?
                   ^
                   |
                   v
str_val: abcdefzzzz\0

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