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

[Eric W. Biederman]

Kees Cook <keescook@xxxxxxxxxxxx> writes:

> On Tue, May 19, 2020 at 02:03:23PM -0500, Eric W. Biederman wrote:
>> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>> 
>> > On Mon, May 18, 2020 at 07:31:14PM -0500, Eric W. Biederman wrote:
>> >> [...]
>> >> 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'm thinking of an LSM writing reading these comments to decide what
> they need to do to the flags, so it's a direction to them to set it to
> true if they have determined that privilege was gained. (Though in
> theory, this is all moot since only the commoncap hook cares.)

The comments for an LSM writer are in include/linux/lsm_hooks.h

 * @bprm_repopulate_creds:
 *	Assuming that the relevant bits of @bprm->cred->security have been
 *	previously set, examine @bprm->file and regenerate them.  This is
 *	so that the credentials derived from the interpreter the code is
 *	actually going to run are used rather than credentials derived
 *	from a script.  This done because the interpreter binary needs to
 *	reopen script, and may end up opening something completely different.
 *	This hook may also optionally check permissions (e.g. for
 *	transitions between security domains).
 *	The hook must set @bprm->active_secureexec to 1 if AT_SECURE should be set to
 *	request libc enable secure mode.
 *	@bprm contains the linux_binprm structure.
 *	Return 0 if the hook is successful and permission is granted.

I hope that is detailed enough.

I will leave the rest of the comments for the maintainer of the code.

I really don't think we should duplicate the prescriptive comments in
multiple locations.

Eric