Re: [PATCH v0] RISC-V: Use Zkr to seed KASLR base address

Conor Dooley
2024-06-10T09:02:53Z

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On Mon, Jun 10, 2024 at 10:33:34AM +0200, Clément Léger wrote:
>
>
> On 07/06/2024 20:51, Deepak Gupta wrote:
> > On Mon, Jun 03, 2024 at 01:47:40PM +0100, Conor Dooley wrote:
> >> On Mon, Jun 03, 2024 at 11:14:49AM +0200, Alexandre Ghiti wrote:
> >>> Hi Conor,
> >>>
> >>> On 31/05/2024 19:31, Conor Dooley wrote:
> >>> > On Fri, May 31, 2024 at 12:23:27PM -0400, Jesse Taube wrote:
> >>> > > Dectect the Zkr extension and use it to seed the kernel base
> >>> address.
> >>> > >
> >>> > > Detection of the extension can not be done in the typical
> >>> fashion, as
> >>> > > this is very early in the boot process. Instead, add a trap handler
> >>> > > and run it to see if the extension is present.
> >>> > You can't rely on the lack of a trap meaning that Zkr is present
> >>> unless
> >>> > you know that the platform implements Ssstrict. The CSR with that
> >>> number
> >>> > could do anything if not Ssstrict compliant, so this approach gets a
> >>> > nak from me. Unfortunately, Ssstrict doesn't provide a way to detect
> >>> > it, so you're stuck with getting that information from firmware.
> >>>
> >>>
> >>> FYI, this patch is my idea, so I'm the one to blame here :)
> >>>
> >>>
> >>> >
> >>> > For DT systems, you can actually parse the DT in the pi, we do it
> >>> to get
> >>> > the kaslr seed if present, so you can actually check for Zkr. With
> >>> ACPI
> >>> > I have no idea how you can get that information, I amn't an ACPI-ist.
> >>>
> >>>
> >>> I took a look at how to access ACPI tables this early when
> >>> implementing the
> >>> Zabha/Zacas patches, but it seems not possible.
> >>>
> >>> But I'll look into this more, this is not the first time we need the
> >>> extensions list very early and since we have no way to detect the
> >>> presence
> >>> of an extension at runtime, something needs to be done.
> >>
> >> Aye, having remembered that reading CSR_SEED could have side-effects on a
> >> system with non-conforming extensions, it'd be good to see if we can
> >> actually do this via detection on ACPI - especially for some other
> >> extensions that we may need to turn on very early (I forget which ones we
> >> talked about this before for). I didn't arm64 do anything with ACPI in
> >> the
> >> pi code, is the code arch/x86/boot/compressed run at an equivilent-ish
> >> point
> >> in boot?
> >
> > cc: +Clement and Atish
> >
> > I don't know all the details but on first glance it seems like instead
> > of ACPI,
> > may be FWFT is a better place for discovery ?
> > https://lists.riscv.org/g/tech-prs/topic/patch_v12_add_firmware/106479571
>
> IMHO, doing discovery in FWFT is not the goal of this extension. I think
> the "real" solution would be to wait for the unified discovery task
> group to come up with something for that (which is their goal I think) [1]

I'm curious to see how that works out. The proposal documents an m-mode
csr, so we'd have to smuggle the information to s-mode somehow...

> Link: https://github.com/riscv/configuration-structure [1]

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