On Wed, Jul 13, 2016 at 09:26:39AM +0100, Russell King - ARM Linux wrote:

On Wed, Jul 13, 2016 at 05:55:33PM +1000, Stewart Smith wrote:

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Russell King - ARM Linux [off-list ref] writes:

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On Wed, Jul 13, 2016 at 02:59:51PM +1000, Stewart Smith wrote:

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Russell King - ARM Linux [off-list ref] writes:

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On Tue, Jul 12, 2016 at 10:58:05PM +0200, Petr Tesarik wrote:

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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.

Ahh.. I missed the bootloader bit there.

In which case, we're the same on OpenPOWER, there is no intermediate
bootloader - in our case we have linux (with kexec) taking on what uboot
or grub is typically used for on other platforms.

Indeed - maybe Eric knows better, but I can't see any situation where
the dtb we load via kexec should ever affect "the bootloader", unless
the "kernel" that's being loaded into kexec is "the bootloader".

Now, going back to the more fundamental issue raised in my first reply,
about the kernel command line.

On x86, I can see that it _is_ possible for userspace to specify a
command line, and the kernel loading the image provides the command
line to the to-be-kexeced kernel with very little checking.  So, if
your kernel is signed, what stops the "insecure userspace" loading
a signed kernel but giving it an insecure rootfs and/or console?

It is not kexec specific. I could do this for regular boot too, right?

Command line options are not signed. I thought idea behind secureboot
was to execute only trusted code and command line options don't enforce
you to execute unsigned code.

So it sounds like different class of security problems which you are
referring to and not necessarily covered by secureboot or signed
kernel.

Vivek