linux-kernel - Re: [PULL] modules-next

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Date:	Mon, 01 Aug 2016 20:11:27 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Cc:	lkml <linux-kernel@...r.kernel.org>, Jessica Yu <jeyu@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Kees Cook <keescook@...omium.org>,
	Libor Pechacek <lpechacek@...e.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PULL] modules-next

On Sun, 2016-07-31 at 21:44 -0400, Linus Torvalds wrote:
> So this feels wrong to me, can you guys please explain:
> 
> On Sun, Jul 31, 2016 at 9:02 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> > 
> > Ben Hutchings (3):
> >       module: Invalidate signatures on force-loaded modules
> >       module: Disable MODULE_FORCE_LOAD when MODULE_SIG_FORCE is enabled
> 
> forcing a load and SIG_FORCE are entirely independent issues, afaik. I
> think requiring signed modules is just a good idea. But that doesn't
> necessarily mean that you don't have a signed module that is signed
> with a key you trust, but you still want to force-load it for the
> wrong kernel version (ie maybe you have a binary-only module from your
> IT department (and your IT department is evil,but at least they sign
> it to show that the module is trust-worthy as coming from them, even
> if they have some dubious behavior), but you did some kernel updates
> that still allow the module to work but the version doesn't match any
> more).

We discussed this before and I thought you were happy with this
version.  If the use case you describe is at all common, it could
perhaps be handled by having a tool that patches the version
information and re-signs the module with a different trusted key.

> Am I missing something? What's the connection between
> MODULE_FORCE_LOAD and MODULE_SIG_FORCE? Because it smells like they
> are independent and that the above changes are very very dubious.

As I understand it:
- module signature enforcement means that root is not trusted to load
  arbitrary code into the kernel; instead the code has to be approved
  by one of the signing key holders
- force-loading a module means "I promise that this module is ABI
  compatible, even though it doesn't appear to be"

No-one signs that promise, and if it's false, the ABI differences could
mean that an otherwise benign module would compromise the kernel.  So
as I see it, the kernel should not trust a force-loaded signed module
any more than an unsigned module.

If you still think that module signature enforcement is compatible with
force-loading, I would like to know what you consider the purpose of
enforcement to be.

Ben.

> I didn't actually pull the tree, I just reacted to the pull request itself.
> 
>              Linus
-- 

Ben Hutchings
Sturgeon's Law: Ninety percent of everything is crap.

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