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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 13 May 2020 12:51:59 -0700
Message-ID: <CAHk-=wjgYDMabsEDW_W3iU0jD8RhndQAMhR2HSMkMZ3rTCQbVg@mail.gmail.com>
Subject: Re: [PATCH 3/5] exec: Remove recursion from search_binary_handler
To:     Rob Landley <rob@landley.net>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.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>,
        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>
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On Tue, May 12, 2020 at 7:32 PM Rob Landley <rob@landley.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