linux-kernel - Re: [PATCH net-next] modules: allow modprobe load regular elf binaries

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Message-ID: <CALCETrVUOToH5WnZMdpk+Vt=NK5OuE=ha62sj7cN5Hw5=qrgRw@mail.gmail.com>
Date:   Fri, 9 Mar 2018 01:04:39 +0000
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexei Starovoitov <ast@...com>
Cc:     Kees Cook <keescook@...omium.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Djalal Harouni <tixxdz@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...com,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH net-next] modules: allow modprobe load regular elf binaries

On Fri, Mar 9, 2018 at 12:57 AM, Alexei Starovoitov <ast@...com> wrote:
> On 3/8/18 4:24 PM, Kees Cook wrote:
>>
>> As Andy asked earlier, why not DYN too to catch PIE executables? Seems
>> like forcing the userspace helper to be non-PIE would defeat some of
>> the userspace defenses in use in most distros.
>
>
> because we don't add features without concrete users.

I disagree here.  If you're going to add a magic trick that triggers
an execve(), then I think you should either support *both* standard,
widely-used types of ELF programs or you should give a compelling use
case that works for ET_EXEC but not for ET_DYN.  Keep in mind that
many distros have a very strong preference for ET_DYN.

Or you could argue that ET_DYN requires tooling changes, but I think
it's awkward to ask the tooling to change in advance of the kernel
being willing to actually invoke the thing.  I'm not actually
convinced that any tooling changes would be needed, though.