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

[Eric W. Biederman]

ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:

> Petr Tesarik <ptesarik@xxxxxxx> writes:
>
>> On Tue, 12 Jul 2016 13:25:11 -0300
>> Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxxxxxxx> wrote:
>>
>>> Hi Eric,
>>> 
>>> I'm trying to understand your concerns leading to your nack. I hope you 
>>> don't mind expanding your thoughts on them a bit.
>>> 
>>> Am Dienstag, 12 Juli 2016, 08:25:48 schrieb Eric W. Biederman:
>>> > AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx> writes:
>>> > > Device tree blob must be passed to a second kernel on DTB-capable
>>> > > archs, like powerpc and arm64, but the current kernel interface
>>> > > lacks this support.
>>> > > 
>>> > > This patch extends kexec_file_load system call by adding an extra
>>> > > argument to this syscall so that an arbitrary number of file descriptors
>>> > > can be handed out from user space to the kernel.
>>> > > 
>>> > > See the background [1].
>>> > > 
>>> > > Please note that the new interface looks quite similar to the current
>>> > > system call, but that it won't always mean that it provides the "binary
>>> > > compatibility."
>>> > > 
>>> > > [1] http://lists.infradead.org/pipermail/kexec/2016-June/016276.html
>>> > 
>>> > So this design is wrong.  The kernel already has the device tree blob,
>>> > you should not be extracting it from the kernel munging it, and then
>>> > reinserting it in the kernel if you want signatures and everything to
>>> > pass.
>>> 
>>> I don't understand how the kernel signature will be invalidated. 
>>> 
>>> There are some types of boot images that can embed a device tree blob in 
>>> them, but the kernel can also be handed a separate device tree blob from 
>>> firmware, the boot loader, or kexec. This latter case is what we are 
>>> discussing, so we are not talking about modifying an embedded blob in the 
>>> kernel image.
>>> 
>>> > What x86 does is pass it's equivalent of the device tree blob from one
>>> > kernel to another directly and behind the scenes.  It does not go
>>> > through userspace for this.
>>> > 
>>> > Until a persuasive case can be made for going around the kernel and
>>> > probably adding a feature (like code execution) that can be used to
>>> > defeat the signature scheme I am going to nack this.
>>> 
>>> I also don't understand what you mean by code execution. How does passing a 
>>> device tree blob via kexec enables code execution? How can the signature 
>>> scheme be defeated?
>>
>> 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.
>>
>> In general, tampering with the hardware inventory of a machine opens up
>> a security hole, and one must be very cautious which modifications are
>> allowed. You're giving this power to an (unsigned, hence untrusted)
>> userspace application; Eric argues that only the kernel should have
>> this power.
>
> At the very least it should be signed.  And of course the more signed
> images we have in different combinations the more easily someone can
> find a combination that does things the people performing the signing
> didn't realizing they were allowing.
>
> So if we can not add an extra variable into the mix it would be good.

But it is even more than that.  There was a giant design discussion that
lasted months before this code was added on x86.  The facts on the
ground on x86 are substantially similar to ARM64.  So coming up and
saying that oh that design sucks and we want to do something completely
different to achieve the exact same goals and then not even discussing
why the current design can not work in the problem descriptions is
inconsiderate.

Not taking the time to understand how something works and why and then
asking people to explain to them what they are doing wrong is rude.  It
is a waste of everyones time.

I thought I had said something to that effect, but it doesn't look like
I did.  Apologies for not being clear about that.

I have had a lot of that this last little while.  Code with big design
issues that really should be justified given people are aiming to
overturn previous design decisions and not even considering those
previous decisions, and I find it quite tiring.

Especially when we are dealing with design decisions with a security
impact, and peopel want to add code to achieve a security goal I expect
people to be paying attention to what effect their changes have on the
entire ecosystem.  Not just saying the current behavior is inconvinient
and using that as a rational for changing things.

Eric