linux-kernel - Re: [PATCH 3/5] exec: Remove recursion from search_binary

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Message-ID: <CAHk-=wjgYDMabsEDW_W3iU0jD8RhndQAMhR2HSMkMZ3rTCQbVg@mail.gmail.com>
Date:   Wed, 13 May 2020 12:51:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Rob Landley <rob@...dley.net>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel <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>,
        LSM List <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 3/5] exec: Remove recursion from search_binary_handler

On Tue, May 12, 2020 at 7:32 PM Rob Landley <rob@...dley.net> wrote:
>
> On 5/12/20 7:20 PM, Linus Torvalds wrote:
> > Ack. I think the AT_EXECFD thing is a sign that this isn't internal to
> > binfmt_misc, but it also shouldn't be gating this issue. In reality,
> > ELF is the only real binary format that matters - the script/misc
> > binfmts are just indirection entries - and it supports AT_EXECFD, so
> > let's just ignore the theoretical case of "maybe nobody exposes it".
>
> Would this potentially make the re-exec-yourself case easier to do at some
> point? (Which nommu needs to do, and /proc/self/exe isn't always available.)

AT_EXECFD may be an ELF thing, but normal ELF binaries don't do that
"we have a fd". So it only triggers for binfmt_misc (and only when the
flag is set for "I want the fd").

So no, this wouldn't help re-exec-yourself in general.

Although I guess we could add an ELF section note that does that whole
"executable fd" thing for other things too.

Everything is possible in theory..

               Linus