[PATCH v2 3/8] exec: Convert security_bprm_set_creds into security_bprm_repopulate_creds

[Eric W. Biederman]

Kees Cook <keescook at chromium.org> writes:

> On Mon, May 18, 2020 at 07:31:14PM -0500, Eric W. Biederman wrote:
>> 
>> Rename bprm->cap_elevated to bprm->active_secureexec and initialize it
>> in prepare_binprm instead of in cap_bprm_set_creds.  Initializing
>> bprm->active_secureexec in prepare_binprm allows multiple
>> implementations of security_bprm_repopulate_creds to play nicely with
>> each other.
>> 
>> Rename security_bprm_set_creds to security_bprm_reopulate_creds to
>> emphasize that this path recomputes part of bprm->cred.  This
>> recomputation avoids the time of check vs time of use problems that
>> are inherent in unix #! interpreters.
>> 
>> In short two renames and a move in the location of initializing
>> bprm->active_secureexec.
>
> I like this much better than the direct call to the capabilities hook.
> Thanks!
>
> Reviewed-by: Kees Cook <keescook at chromium.org>
>
> One nit is a bikeshed on the name "active_secureexec", since
> the word "active" isn't really associated with any other part of the
> binfmt logic. It's supposed to be "latest state from the binfmt loop",
> so instead of "active", I considered these words that I also didn't
> like: "current", "this", "recent", and "now". Is "latest" better than
> "active"? Probably not.

I had pretty much the same problem.  Active at least conveys that it
is still malleable and might change.

>> [...]
>> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
>> index d1217fcdedea..8605ab4a0f89 100644
>> --- a/include/linux/binfmts.h
>> +++ b/include/linux/binfmts.h
>> @@ -27,10 +27,10 @@ struct linux_binprm {
>>  	unsigned long argmin; /* rlimit marker for copy_strings() */
>>  	unsigned int
>>  		/*
>> -		 * True if most recent call to cap_bprm_set_creds
>> +		 * True if most recent call to security_bprm_set_creds
>>  		 * resulted in elevated privileges.
>>  		 */
>> -		cap_elevated:1,
>> +		active_secureexec:1,
>
> Also, I'd like it if this comment could be made more verbose as well, for
> anyone trying to understand the binfmt execution flow for the first time.
> Perhaps:
>
> 		/*
> 		 * Must be set True during the any call to
> 		 * bprm_set_creds hook where the execution would
> 		 * reuslt in elevated privileges. (The hook can be
> 		 * called multiple times during nested interpreter
> 		 * resolution across binfmt_script, binfmt_misc, etc).
> 		 */
Well it is not during but after the call that it becomes true.
I think most recent covers the case of multiple calls.

I think having the loop explicitly in the code a few patches
later makes it clear that there is a loop dealing with interpreters.

Conciseness has a virtue in that it is easy to absorb.  Seeing
active says most recent and secureexec does not is enough to ask
questions and look at the code.

Eric