Re: [RFC 0/3] extend kexec_file_load system call

[Ard Biesheuvel]

On 13 July 2016 at 09:36, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxx> wrote:
> On Wed, Jul 13, 2016 at 02:59:51PM +1000, Stewart Smith wrote:
>> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> 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.
>

The particular example may not apply, but the argument that the DTB
-as a description of the hardware topology- needs to be signed if the
kernel is also signed is valid. We do the same in the UEFI stub, i.e.,
it normally takes a dtb= argument to allow the DTB to be overridden,
but this feature is disabled when Secure Boot is in effect. By the
same reasoning, if any kind of kexec kernel image validation is in
effect, we should either validate the DTB image as well, or disallow
external DTBs and only perform kexec with the kernel's current DTB
(the blob it was booted with, not the unflattened data structure)