linux-kernel - Re: [PATCH v2 3/8] exec: Convert security_bprm_set_creds into security_bprm_repopulate

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Message-ID: <202005191111.9B389D33@keescook>
Date:   Tue, 19 May 2020 11:21:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-security-module@...r.kernel.org,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2 3/8] exec: Convert security_bprm_set_creds into
 security_bprm_repopulate_creds

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@...omium.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.

> [...]
> 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).
		 */


-- 
Kees Cook