Re: [kernel-hardening] [PATCH v7 0/9] x86/mm: memory area address KASLR

[Ard Biesheuvel]

On 23 June 2016 at 21:58, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Thu, Jun 23, 2016 at 12:33 PM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
>> Hey Kees, Thomas,
>>
>> On Wed, Jun 22, 2016 at 10:05:51AM -0700, Kees Cook wrote:
>>> On Wed, Jun 22, 2016 at 8:59 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>>> > On Wed, Jun 22, 2016 at 5:47 AM, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
>>> >> Hey Kees,
>>> >>
>>> >> On Tue, Jun 21, 2016 at 05:46:57PM -0700, Kees Cook wrote:
>>> >>> Notable problems that needed solving:
>>> >> ...
>>> >>>  - Reasonable entropy is needed early at boot before get_random_bytes()
>>> >>>    is available.
>>> >>
>>> >> This series is targetting x86, which typically has RDRAND/RDSEED
>>> >> instructions.  Are you referring to other arches?  Older x86?  Also,
>>> >> isn't this the same requirement for base address KASLR?
>>> >>
>>> >> Don't get me wrong, I want more diverse entropy sources available
>>> >> earlier in the boot process as well. :-)  I'm just wondering what's
>>> >> different about this series vs base address KASLR wrt early entropy
>>> >> sources.
>>> >>
>>> >
>>> > I think Kees was referring to the refactor I did to get the similar
>>> > entropy generation than KASLR module randomization. Our approach was
>>> > to provide best entropy possible even if you have an older processor
>>> > or under virtualization without support for these instructions.
>>> > Unfortunately common on companies with a large number of older
>>> > machines.
>>>
>>> Right, the memory offset KASLR uses the same routines as the kernel
>>> base KASLR. The issue is with older x86 systems, which continue to be
>>> very common.
>>
>> We have the same issue in embedded. :-(  Compounded by the fact that
>> there is no rand instruction (at least not on ARM).  So, even if there's
>> a HW-RNG, you can't access it until the driver is loaded.
>>
>> This is compounded by the fact that most systems deployed today have
>> bootloaders a) without hw-rng drivers, b) without dtb editing, and c)
>> without dtb support at all.
>>
>> My current thinking is to add a devicetree property
>> "userspace,random-seed" <address, len>.  This way, existing, deployed
>> boards can append a dtb to a modern kernel with the property set.
>> The factory bootloader then only needs to amend its boot scripts to read
>> random-seed from the fs to the given address.
>
> The arm64 KASLR implementation has defined a way for boot loaders to
> pass in an seed similar to this. It might be nice to have a fall-back
> to a DT entry, though, then the bootloaders don't need to changed.
>
> Ard might have some thoughts on why DT wasn't used for KASLR (I assume
> the early parsing overhead, but I don't remember the discussion any
> more).
>

On arm64, only DT is used for KASLR (even when booting via ACPI). My
first draft used register x1, but this turned out to be too much of a
hassle, since parsing the DT is also necessary to discover whether
there is a 'nokaslr' argument on the kernel command line. So the
current implementation only supports a single method, which is the
/chosen/kaslr-seed uint64 property.

>> Modern systems that receive a seed from the bootloader via the
>> random-seed property (typically from the hw-rng) can mix both sources
>> for increased resilience.
>
> Yeah, that could work.
>
>> Unfortunately, I'm not very familiar with the internals of x86
>> bootstrapping.  Could GRUB be scripted to do a similar task?  How would
>> the address and size of the seed be passed to the kernel?  command line?
>
> Command line could work (though it would need scrubbing to avoid it
> leaking into /proc/cmdine), but there's also the "zero-page" used by
> bootloaders to pass details to the kernel (see
> Documentation/x86/boot.txt). Right now, x86 has sufficient entropy
> (though rdrand is best).
>
> -Kees
>
> --
> Kees Cook
> Chrome OS & Brillo Security