Re: [GIT PULL] Kernel lockdown for secure boot

[Andy Lutomirski]

On Tue, Apr 3, 2018 at 11:45 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, Apr 3, 2018 at 9:45 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> On Tue, Apr 3, 2018 at 9:29 AM, Matthew Garrett <mjg59@xxxxxxxxxx> wrote:
>>> On Tue, Apr 3, 2018 at 8:11 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>> Can you explain that much more clearly?  I'm asking why booting via
>>>> UEFI Secure Boot should enable lockdown, and I don't see what this has
>>>> to do with kexec.  And "someone blacklist[ing] your key in the
>>>> bootloader" sounds like a political issue, not a technical issue.
>>>
>>> A kernel that allows users arbitrary access to ring 0 is just an
>>> overfeatured bootloader. Why would you want secure boot in that case?
>>
>> To get a chain of trust.  I can provision a system with some public
>> keys, stored in UEFI authenticated variables, such that the system
>> will only boot a signed image.  That signed image, can, in turn, load
>> a signed (or hashed or otherwise verfified) kernel and a verified
>> initramfs.  The initramfs can run a full system from a verified (using
>> dm-verity or similar) filesystem, for example.  Now it's very hard to
>> persistently attack this system.  Chromium OS does something very much
>> like this, except that it doesn't use UEFI as far as I know.  So does
>> iOS, and so do some Android versions.
>
> Correct, Chrome OS does not use UEFI, and we still want this patch
> series, as it plugs all the known "intentional" escalation paths from
> uid-0 to ring-0. Happily, that means all the politics around the UEFI
> and Secure Boot case can be ignored, because those issues are specific
> to Secure Boot, not the lockdown series. (They are _related_, sure,
> but lockdown isn't only about Secure Boot -- it's just that SB is one
> of the widely deployed implementations of this kind of
> trust-chain-booting-thing. Chrome OS and Android's Verified Boot do
> similar things and have the same expectations about the uid-0/ring-0
> separation.)
>
> The goal for that bright line on Chrome OS and Android is to stop
> attack persistence. We want to know that a reboot onto a new kernel
> and OS image will actually result in getting the desired system state,
> and that any attack on persistent system data (even for things running
> with full root privileges) can't result in using kernel interfaces to
> gain kernel control. This isn't expected to be _perfect_, since
> nothing is. But it creates a place to work from. The idea that uid-0
> is NOT ring-0 is still relatively new, so the existing designs in the
> kernel aren't well suited to building that distinction. I view this
> series as a solid first step to getting there, though.
>

But wouldn't Chrome OS possibly want to lock down kernel memory write
vectors but not read vectors?  After all, debugging is useful even on
Chrome OS.

--Andy