linux-kernel - Re: [RFC 0/3] extend kexec_file

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Message-ID: <20160713073657.GX1041@n2100.armlinux.org.uk>
Date:	Wed, 13 Jul 2016 08:36:57 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Stewart Smith <stewart@...ux.vnet.ibm.com>
Cc:	Petr Tesarik <ptesarik@...e.cz>,
	linux-arm-kernel@...ts.infradead.org, bhe@...hat.com,
	arnd@...db.de, linuxppc-dev@...ts.ozlabs.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
	dyoung@...hat.com, vgoyal@...hat.com
Subject: Re: [RFC 0/3] extend kexec_file_load system call

On Wed, Jul 13, 2016 at 02:59:51PM +1000, Stewart Smith wrote:
> Russell King - ARM Linux <linux@...linux.org.uk> writes:
> > On Tue, Jul 12, 2016 at 10:58:05PM +0200, Petr Tesarik wrote:
> >> I'm not an expert on DTB, so I can't provide an example of code
> >> execution, but you have already mentioned the /chosen/linux,stdout-path
> >> property. If an attacker redirects the bootloader to an insecure
> >> console, they may get access to the system that would otherwise be
> >> impossible.
> >
> > I fail to see how kexec connects with the boot loader - the DTB image
> > that's being talked about is one which is passed from the currently
> > running kernel to the to-be-kexec'd kernel.  For ARM (and I suspect
> > also ARM64) that's a direct call chain which doesn't involve any
> > boot loader or firmware, and certainly none that would involve the
> > passed DTB image.
> 
> For OpenPOWER machines, kexec is the bootloader. Our bootloader is a
> linux kernel and initramfs with a UI (petitboot) - this means we never
> have to write a device driver twice: write a kernel one and you're done
> (for booting from the device and using it in your OS).

I think you misunderstood my point.

On ARM, we do not go:

	kernel (kexec'd from) -> boot loader -> kernel (kexec'd to)

but we go:

	kernel (kexec'd from) -> kernel (kexec'd to)

There's no intermediate step involving any bootloader.

Hence, my point is that the dtb loaded by kexec is _only_ used by the
kernel which is being kexec'd to, not by the bootloader, nor indeed
the kernel which it is loaded into.

Moreover, if you read the bit that I quoted (which is what I was
replying to), you'll notice that it is talking about the DTB loaded
by kexec somehow causing the _bootloader_ to be redirected to an
alternative console.  This point is wholely false on ARM.

-- 
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