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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 9 May 2020 13:16:53 -0700
Message-ID: <CAHk-=wg=0nAA9qGAogiFjpf0oCDcjkQE-u6umWCW-PkhrC8jrQ@mail.gmail.com>
Subject: Re: [PATCH 3/5] exec: Remove recursion from search_binary_handler
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        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>
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On Sat, May 9, 2020 at 12:45 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> Instead of recursing in search_binary_handler have the methods that
> would recurse return a positive value, and simply loop in exec_binprm.
>
> This is a trivial change as all of the methods that would recurse do
> so as effectively the last thing they do.   Making this a trivial code
> change.

Looks good.

I'd suggest doing that loop slightly differently:

> -       ret = search_binary_handler(bprm);
> +       do {
> +               depth++;
> +               ret = search_binary_handler(bprm);
> +               /* This allows 4 levels of binfmt rewrites before failing hard. */
> +               if ((ret > 0) && (depth > 5))
> +                       ret = -ELOOP;
> +       } while (ret > 0);
>          if (ret >= 0) {

That's really an odd way to write this.

So honestly, if "ret < 0", then we can just return directly.

So I think would make much more sense to do this loop something like

        for (depth = 0; depth < 5; depth++) {
                int ret;

                ret = search_binary_handler(bprm);
                if (ret < 0)
                        return ret;

                /* Continue searching for the next binary handler? */
                if (ret > 0)
                        continue;

                /* Success! */
                audit_bprm(bprm);
                trace_sched_process_exec(current, old_pid, bprm);
                ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
                proc_exec_connector(current);
                return 0;
        }
        return -ELOOP;

(if I read the logic of exec_binprm() right - I might have missed something).

                Linus