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Date:   Tue, 12 May 2020 16:08:56 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.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 <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>,
        LSM List <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 3/5] exec: Remove recursion from search_binary_handler
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On Tue, May 12, 2020 at 03:31:57PM -0500, Eric W. Biederman wrote:
> >> It is possible although unlikely for userspace to find the file
> >> descriptor without consulting AT_EXECFD so just to be conservative I
> >> think we should install the file descriptor in begin_new_exec even if
> >> the next interpreter does not support AT_EXECFD.
> >
> > I think universally installing the fd needs to be a distinct patch --
> > it's going to have a lot of consequences, IMO. We can certainly deal
> > with them, but I don't think it should be part of this clean-up series.
> 
> I meant generically installing the fd not universally installing it.
> 
> >> I am still working on how to handle recursive binfmts but I suspect it
> >> is just a matter of having an array of struct files in struct
> >> linux_binprm.
> >
> > If install is left if binfmt_misc, then the recursive problem goes away,
> > yes?
> 
> I don't think leaving the install in binfmt_misc is responsible at this
> point.

I'm nearly certain the answer is "yes", but I wonder if we should stop
for a moment and ask "does anything still use MISC_FMT_OPEN_BINARY ? It
looks like either "O" or "C" binfmt_misc registration flag. My installed
binfmts on Ubuntu don't use them...

I'm currently pulling a list of all the packages in Debian than depend
on the binfmt-support package and checking their flags.

-- 
Kees Cook