Re: [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Mon, May 18, 2020 at 07:33:21PM -0500, Eric W. Biederman wrote:
>> 
>> When replacing loops with next_non_spacetab and next_terminator care
>> has been take that the logic of the parsing code (short of replacing
>> characters by '\0') remains the same.
>
> Ah, interesting. As in, bprm->buf must not be modified unless the binfmt
> handler is going to succeed. I think this requirement should be
> documented in the binfmt struct header file.

I think the best way to document this is to modify bprm->buf to be
"const char buf[BINPRM_BUF_SIZE]" or something like that and not
allow any modifications by anything except for the code that
initially reads in contets of the file.

That unfortunately requires copy_strings_kernel which has become
copy_string_kernel to take a length.  Then I don't need to modify the
buffer at all here.

I believe binfmt_scripts is a bit unique in wanting to modify the buffer
because it is parsing strings.

The requirement is that a binfmt should not modify bprm unless it will
succeed or fail with an error that is not -ENOEXEC.  The fundamental
issue is that search_binary_handler will reuse bprm if -ENOEXEC is
returned.

Until the next patch there is an escape hatch by clearing and closing
bprm->file but that goes away.  Which is why I need this patch.

I guess I can see adding a comment about the general case of not
changing bprm unless you are doing something other than returning
-ENOEXEC and letting the search continue.

Eric


>> [...]
>> diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
>> index 8d718d8fd0fe..85e0ef86eb11 100644
>> --- a/fs/binfmt_script.c
>> +++ b/fs/binfmt_script.c
>> @@ -71,39 +56,48 @@ static int load_script(struct linux_binprm *bprm)
>>  	 * parse them on its own.
>>  	 */
>>  	buf_end = bprm->buf + sizeof(bprm->buf) - 1;
>> -	cp = strnchr(bprm->buf, sizeof(bprm->buf), '\n');
>> -	if (!cp) {
>> -		cp = next_non_spacetab(bprm->buf + 2, buf_end);
>> -		if (!cp)
>> +	i_end = strnchr(bprm->buf, sizeof(bprm->buf), '\n');
>> +	if (!i_end) {
>> +		i_end = next_non_spacetab(bprm->buf + 2, buf_end);
>> +		if (!i_end)
>>  			return -ENOEXEC; /* Entire buf is spaces/tabs */
>>  		/*
>>  		 * If there is no later space/tab/NUL we must assume the
>>  		 * interpreter path is truncated.
>>  		 */
>> -		if (!next_terminator(cp, buf_end))
>> +		if (!next_terminator(i_end, buf_end))
>>  			return -ENOEXEC;
>> -		cp = buf_end;
>> +		i_end = buf_end;
>>  	}
>> -	/* NUL-terminate the buffer and any trailing spaces/tabs. */
>> -	*cp = '\0';
>> -	while (cp > bprm->buf) {
>> -		cp--;
>> -		if ((*cp == ' ') || (*cp == '\t'))
>> -			*cp = '\0';
>> -		else
>> -			break;
>> -	}
>> -	for (cp = bprm->buf+2; (*cp == ' ') || (*cp == '\t'); cp++);
>> -	if (*cp == '\0')
>> +	/* Trim any trailing spaces/tabs from i_end */
>> +	while (spacetab(i_end[-1]))
>> +		i_end--;
>> +
>> +	/* Skip over leading spaces/tabs */
>> +	i_name = next_non_spacetab(bprm->buf+2, i_end);
>> +	if (!i_name || (i_name == i_end))
>>  		return -ENOEXEC; /* No interpreter name found */
>> -	i_name = cp;
>> +
>> +	/* Is there an optional argument? */
>>  	i_arg = NULL;
>> -	for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++)
>> -		/* nothing */ ;
>> -	while ((*cp == ' ') || (*cp == '\t'))
>> -		*cp++ = '\0';
>> -	if (*cp)
>> -		i_arg = cp;
>> +	i_sep = next_terminator(i_name, i_end);
>> +	if (i_sep && (*i_sep != '\0'))
>> +		i_arg = next_non_spacetab(i_sep, i_end);
>> +
>> +	/*
>> +	 * If the script filename will be inaccessible after exec, typically
>> +	 * because it is a "/dev/fd/<fd>/.." path against an O_CLOEXEC fd, give
>> +	 * up now (on the assumption that the interpreter will want to load
>> +	 * this file).
>> +	 */
>> +	if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
>> +		return -ENOENT;
>> +
>> +	/* Release since we are not mapping a binary into memory. */
>> +	allow_write_access(bprm->file);
>> +	fput(bprm->file);
>> +	bprm->file = NULL;
>> +
>>  	/*
>>  	 * OK, we've parsed out the interpreter name and
>>  	 * (optional) argument.
>> @@ -121,7 +115,9 @@ static int load_script(struct linux_binprm *bprm)
>>  	if (retval < 0)
>>  		return retval;
>>  	bprm->argc++;
>> +	*((char *)i_end) = '\0';
>>  	if (i_arg) {
>> +		*((char *)i_sep) = '\0';
>>  		retval = copy_strings_kernel(1, &i_arg, bprm);
>>  		if (retval < 0)
>>  			return retval;
>
> I think this is all correct, though I'm always suspicious of my visual
> inspection of string parsers. ;)
>
> I had a worry the \n was not handled correctly in some case. I.e. before
> any \n was converted into \0, and so next_terminator() didn't need to
> consider \n separately. (next_non_spacetab() doesn't care since \n and \0
> are both not ' ' nor '\t'.) For next_terminator(), though, I was worried
> there was a case where *i_end == '\n', and next_terminator()
> will return NULL instead of "last" due to *last being '\n' instead of
> '\0', causing a problem, but you're using the adjusted i_end so I think
> it's correct. And you've handled i_name == i_end.
>
> I will see if I can find my testing scripts I used when commit
> b5372fe5dc84 originally landed to double-check... until then:
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>