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

[Alexander Monakov <amonakov@ispras.ru>]

On Mon, 4 Jan 2016, Alexander Cherepanov wrote:
> 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]);
> }

True, but it wouldn't solve the problem in its entirety.  There's a similar
issue further down (line 26 atm), where __environ[i] is tested in a loop.
However, if we executed free(__env_map[j]), then __env_map[j]==__environ[i]
had held.  Thus, entering the loop invokes the same UB.

To me the implementation looks weird due to how it restarts scanning __environ
with 'goto again' from position 0 instead of current position. I can propose
the following rewrite (untested):

for (i=0; __environ[i]; i++) {
	char *e = __environ[i];
	if (!memcmp(name, e, l) && e[l] == '=') {
		for (j=i--; __environ[j]; j++)
			__environ[j] = __environ[j+1];
		if (__env_map) {
			for (j=0; __env_map[j] && __env_map[j] != e; j++);
			if (__env_map[j]) {
				free(__env_map[j]);
				do __env_map[j] = __env_map[j+1];
				while (__env_map[++j]);
			}
		}
	}
}

Alexadner