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Date:   Tue, 19 May 2020 12:14:40 -0700
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
Message-ID: <202005191211.97BCF9DA@keescook>
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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.)

> 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.

I still think a hint about the nature of nested exec resolution would be
nice in here somewhere, especially given that this value is zeroed
before each call to the hook.

-- 
Kees Cook