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

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Date:   Mon, 11 May 2020 14:10:23 -0500
From:   Rob Landley <rob@...dley.net>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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>,
        Kees Cook <keescook@...omium.org>,
        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>, dalias@...c.org
Subject: Re: [PATCH 3/5] exec: Remove recursion from search_binary_handler

On 5/11/20 9:33 AM, Eric W. Biederman wrote:
> What I do see is that interp_data is just a parameter that is smuggled
> into the call of search binary handler.  And the next binary handler
> needs to be binfmt_elf for it to make much sense, as only binfmt_elf
> (and binfmt_elf_fdpic) deals with BINPRM_FLAGS_EXECFD.

The binfmt_elf_fdpic driver is separate from binfmt_elf for the same reason
ext2/ext3/ext4 used to have 3 drivers: fdpic is really just binfmt_elf with the
4 main sections (text, data, bss, rodata) able to move independently of each
other (each tracked with its own base pointer).

It's kind of -fPIE on steroids, and various security people have sniffed at it
over the years to give ASLR more degrees of freedom on with-MMU systems. Many
moons ago Rich Felker proposed teaching the fdpic loader how to load normal ELF
binaries so there's just the one loader (there's a flag in the ELF header to say
whether the sections are independent or not).

Rob