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

Conor Dooley
2024-06-03T12:47:44Z

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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?

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