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

From: Stewart Smith <[email-masked]>
Date: Wed, 13 Jul 2016 18:20:10 +1000

Ard Biesheuvel <[email-masked]> writes:
> On 13 July 2016 at 09:36, Russell King - ARM Linux
> <[email-masked]> wrote:
>> On Wed, Jul 13, 2016 at 02:59:51PM +1000, Stewart Smith wrote:
>>> Russell King - ARM Linux <[email-masked]> 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)

DTB booted with != current description of hardware

We could have had: PCI hotplug, CPU/memory/cache offlined due to
hardware error, change in available pstates / CPU frequencies.

There is merit in having a signed dtb if you're booting a signed kernel
in a secure boot scenario. However, we still need to set up /chosen/ and
we still need a way to do something like the offb hack.

-- 
Stewart Smith
OPAL Architect, IBM.