Re: [PATCH] fix use of pointer after free in unsetenv

[Alexander Cherepanov <ch3root@openwall.com>]

On 2016-01-04 06:05, Rich Felker wrote:
> On Mon, Jan 04, 2016 at 02:09:44AM +0300, Alexander Cherepanov wrote:
>> The code in [1] uses a pointer which was freed and hence has an
>> indeterminate value. Patch attached.
>>
>> [1] http://git.musl-libc.org/cgit/musl/tree/src/env/unsetenv.c#n23
>
> The bug sounds a lot scarier than it actually is.

Sure. I have tried at least not to use the term "use after free" but 
mentioning directly that the pointer is not dereferenced would be even 
better.

> I don't think any
> compilers will break this yet but it is indeed UB.

I think so too.

>> >From f446b5811a8abc08bcc8202aa241dce82d4c917d Mon Sep 17 00:00:00 2001
>> From: Alexander Cherepanov <cherepan@mccme.ru>
>> Date: Mon, 4 Jan 2016 01:40:03 +0300
>> Subject: [PATCH] fix use of pointer after free in unsetenv
>>
>> the value of a pointer becomes indeterminate after free() so delay free()
>> until the pointer is not needed anymore.
>>
>> ---
>>   src/env/unsetenv.c |    3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/env/unsetenv.c b/src/env/unsetenv.c
>> index 3569335..b5d8b19 100644
>> --- a/src/env/unsetenv.c
>> +++ b/src/env/unsetenv.c
>> @@ -19,9 +19,10 @@ again:
>>   	if (__environ[i]) {
>>   		if (__env_map) {
>>   			for (j=0; __env_map[j] && __env_map[j] != __environ[i]; j++);
>> -			free (__env_map[j]);
>> +			char *t =__env_map[j];
>>   			for (; __env_map[j]; j++)
>>   				__env_map[j] = __env_map[j+1];
>> +			free (t);
>
> Wouldn't something like this be simpler:
>
> 	do __env_map[j] = __env_map[j+1];
> 	while (__env_map[++j]);

This depends on whether our __env_map[j] could be 0. The condition 
"__env_map[j]" in the previous loop hints that it could. Then it should 
be something like this:

if (__env_map[j]) {
	free (__env_map[j]);
	do __env_map[j] = __env_map[j+1];
	while (__env_map[++j]);
}

-- 
Alexander Cherepanov