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

From: Kees Cook <keescook@chromium.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Greg Ungerer <gerg@linux-m68k.org>, Rob Landley <rob@landley.net>,
	Bernd Edlinger <bernd.edlinger@hotmail.de>,
	linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Casey Schaufler <casey@schaufler-ca.com>,
	linux-security-module@vger.kernel.org,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v2 3/8] exec: Convert security_bprm_set_creds into security_bprm_repopulate_creds
Date: Wed, 20 May 2020 13:53:18 -0700	[thread overview]
Message-ID: <202005201353.6D3B1FD0BF@keescook> (raw)
In-Reply-To: <87wo56gxv5.fsf@x220.int.ebiederm.org>

On Wed, May 20, 2020 at 03:22:38PM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > On Tue, May 19, 2020 at 02:03:23PM -0500, Eric W. Biederman wrote:
> >> Kees Cook <keescook@chromium.org> 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.

Okay, that's fair enough. Thanks!

-- 
Kees Cook