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Date:   Tue, 19 May 2020 12:17:47 -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 4/8] exec: Allow load_misc_binary to call
 prepare_binfmt unconditionally
Message-ID: <202005191215.5411A2D1@keescook>
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On Tue, May 19, 2020 at 02:08:34PM -0500, Eric W. Biederman wrote:
> Kees Cook <keescook@chromium.org> writes:
> 
> > On Mon, May 18, 2020 at 07:31:51PM -0500, Eric W. Biederman wrote:
> >> [...]
> >> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> >> index 8605ab4a0f89..dbb5614d62a2 100644
> >> --- a/include/linux/binfmts.h
> >> +++ b/include/linux/binfmts.h
> >> @@ -26,6 +26,8 @@ struct linux_binprm {
> >>  	unsigned long p; /* current top of mem */
> >>  	unsigned long argmin; /* rlimit marker for copy_strings() */
> >>  	unsigned int
> >> +		/* It is safe to use the creds of a script (see binfmt_misc) */
> >> +		preserve_creds:1,
> >
> > How about:
> >
> > 		/*
> > 		 * A binfmt handler will set this to True before calling
> > 		 * prepare_binprm() if it is safe to reuse the previous
> > 		 * credentials, based on bprm->file (see binfmt_misc).
> > 		 */
> 
> I think that is more words saying less.
> 
> While I agree it might be better.  I don't see what your comment adds to
> the understanding.  What do you see my comment not saying that is important?

I think your comment is aimed at the consumer of preserve_creds (i.e.
the fs/exec.c code), whereas I think the comment should be directed at
a binfmt author, who wants to answer the question "why would I set this
flag?" Though I strongly hope we never have new binfmts. ;)

-- 
Kees Cook