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

[Ard Biesheuvel]

On 24 June 2016 at 03:11, Jason Cooper <jason@xxxxxxxxxxxxxx> wrote:
> Hi Ard,
>
> On Thu, Jun 23, 2016 at 10:05:53PM +0200, Ard Biesheuvel wrote:
>> 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:
>> >> 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.
>
> Ok, just to clarify (after a short offline chat), my goal is to set a
> userspace,random-seed <addr, len> property in the device tree once.
> The bootloader scripts would also only need to be altered once.
>
> Then, at each boot, the bootloader reads the entirety of
> /var/lib/misc/random-seed (512 bytes) into the configured address.
> random-seed could be in /boot, or on a flash partition.
>
> The decompressor would consume a small portion of that seed for kaslr
> and such.  After that, the rest would be consumed by random.c to
> initialize the entropy pools.
>

I see. This indeed has little to do with the arm64 KASLR case, other
than that they both use a DT property.

In the arm64 KASLR case, I deliberately chose to leave it up to the
bootloader/firmware to roll the dice, for the same reason you pointed
out, i.e., that there is no architected way on ARM to obtain random
bits. So in that sense, what you are doing is complimentary to my
work, and a KASLR  aware arm64 bootloader would copy some of its
random bits taken from /var/lib/misc/random-seed into the
/chosen/kaslr-seed DT property. Note that, at the moment, this DT
property is only an internal contract between the kernel's UEFI stub
and the kernel proper, so we could still easily change that if
necessary.

Alternatively, if we go with your solution, the KASLR code should read
from the address in userspace,random-seed rather than the
/chosen/kaslr-seed property itself. (or use the former as a fallback
if the latter was not found)

-- 
Ard.