linux-kernel - Re: [PATCH V3 08/11] kexec: Disable at runtime if the kernel enforces module loading restrictions

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Date:	Sun, 08 Sep 2013 10:22:55 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Matthew Garrett <matthew.garrett@...ula.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH V3 08/11] kexec: Disable at runtime if the kernel
 enforces module loading restrictions

On Sun, 2013-09-08 at 17:15 +0000, Matthew Garrett wrote:
> On Sun, 2013-09-08 at 10:11 -0700, James Bottomley wrote:
> 
> > That's not true if you look at the use cases.  Distros use signed
> > modules to taint the kernel:  insert an unsigned one and the kernel
> > taints; insert a properly signed one and it doesn't.  They use it for
> > support to tell if you've been adhering to your contract.  That use case
> > has nothing to do with security.
> 
> That use case has nothing to do with this patch, either. It's completely
> unaffected. This only triggers if the kernel is configured to refuse the
> loading of unsigned modules.
> 
> > The analogy is rubbish.  I can give away CAP_SYS_MODULE and enforce what
> > modules those I've given the permission to can insert by signing them.
> > I keep CAP_SYS_BOOT, so they can't use kexec to subvert this.
> 
> Yeah, that's a good argument for why capabilities are mostly pointless.
> If I have CAP_SYS_BOOT I can give myself any other capabilities. Why
> bother?

It's an argument that CAP_SYS_BOOT is too powerful yes, but if you
recall, I said I keep that one.  In the rather lame analogy, what I do
by giving away CAP_SYS_MODULE and enforcing module signing while keeping
CAP_SYS_BOOT is allow people into my conservatory to play with the
plants but not into my house to steal the silver ... saying CAP_SYS_BOOT
is too powerful doesn't affect that use case because I haven't given
away CAP_SYS_BOOT.

James

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