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

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Message-Id: <20180308.213153.2003279953084099668.davem@davemloft.net>
Date:   Thu, 08 Mar 2018 21:31:53 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     luto@...nel.org
Cc:     alexei.starovoitov@...il.com, keescook@...omium.org,
        ast@...nel.org, tixxdz@...il.com, viro@...iv.linux.org.uk,
        daniel@...earbox.net, torvalds@...ux-foundation.org,
        gregkh@...uxfoundation.org, mcgrof@...nel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-api@...r.kernel.org
Subject: Re: [PATCH net-next] modules: allow modprobe load regular elf
 binaries

From: Andy Lutomirski <luto@...nel.org>
Date: Fri, 9 Mar 2018 02:12:24 +0000

> First, compile your user code and emit a staitc binary.  Use objdump
> fiddling or a trivial .S file to make that static binary into a
> variable.  Then write a tiny shim module like this:
> 
> extern unsigned char __begin_user_code[], __end_user_code[];
> 
> int __init init_shim_module(void)
> {
>   return call_umh_blob(__begin_user_code, __end_user_code - __begin_user_code);
> }
> 
> By itself, this is clearly a worse solution than yours, but it has two
> benefits, one small and two big.  The small benefit is that it is
> completely invisible to userspace: the .ko file is a bona fide module.

Anything you try to do which makes these binaries "special" is a huge
negative.

> The big benefits are:

I don't see those things as benefits at all, and Alexei's scheme can
easily be made to work in your benefit #1 case too.

It's a user binary.  It's shipped with the kernel and it's signed.

If we can't trust that, we can't trust much else.

And this whole container argument..  It's a mirage.

Kernel modules are 1000 times worse, since they can access any
container and any namespace they want.