[Query] ARM64 kaslr support - randomness, seeding and kdump

AKASHI Takahiro
2018-04-09T04:31:39Z

Hi,

On Mon, Apr 09, 2018 at 09:31:34AM +0530, Bhupesh Sharma wrote:
> Hi Akashi,
>
> On Fri, Apr 6, 2018 at 7:39 AM, AKASHI Takahiro
> <takahiro.akashi@linaro.org> wrote:
> > Bhupesh,
> >
> > On Fri, Mar 16, 2018 at 03:05:10PM +0530, Bhupesh Sharma wrote:
> >> On Wed, Mar 14, 2018 at 11:54 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> >> > On Wed, Mar 14, 2018 at 11:10:53AM +0900, AKASHI Takahiro wrote:
> >> >> If kaslr-seed has a critical value in terms of security, is kexec-tools
> >> >> a right place? It is exposed to user space albeit for a short time of period.
> >> >
> >> > The kernel zeroes the seed in the DT at boot time, so the current seed
> >> > isn't visible to userspace.
> >> >
> >> > If kexec-tools generates a seed, and inserts it into the DTB that it
> >> > loads, this is only visible to kexec tools or other applications which
> >> > can inspect its memory, so I don't think this is much of a concern.
> >> > Anything with such privilege can presumably kexec() to arbitrary code
> >> > anyhow.
> >> >
> >> > The next kernel will then zero its seed in the DT at boot time, so
> >> > similarly this won't be visible to userspace on the new kernel.
> >> >
> >> > FWIW, having kexec tools generate a seed for the kexec_load() case makes
> >> > sense to me.
> >>
> >> Fair enough. I will try to take a stab at the same and come back with
> >> my findings on this thread.
> >
> > How's your progress here?
>
> I am almost done with the implementation.
> Unfortunately I lost most of the last week trying to revive my arm64
> board (which supports
> EFI_RNG_PROTOCOL and hence can be used to test the kaslr-seed related
> stuff), so I was not
> able to test the implementation.
>
> Now that the board is up, I think I can test and thrash out any
> missing clogs in the approach.

Sounds good.

> > I've already added kaslr support (i.e. "virtual randomisation") to
> > my kexec_file patch set.
> > # just a few lines of code, though
>
> Hmm, have you sent out a new version already (kexec_file_load), as the last
> version in my inbox still mentions in the cover letter that we need a
> EFI stub like approach
> to really support CONFIG_RANDOMIZE_BASE. Or, am I missing something?

No, not yet.
While I've also added some sort of "physical randomisation",
I'd like to put my post on hold until v4.17-rc1.

> I would love to have a look at the patch and try it at my end, so
> could you please share
> a pointer to the same.

Your test will be very much appreciated.

Thanks,
-Takahiro AKASHI
>
> Regards,
> Bhupesh