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

[Deepak Gupta]

On Mon, Jun 10, 2024 at 10:56:35PM +0100, Conor Dooley wrote:
> On Mon, Jun 10, 2024 at 02:06:50PM -0700, Deepak Gupta wrote:
> > On Mon, Jun 10, 2024 at 11:16:42AM +0200, Clément Léger wrote:
> > > On 10/06/2024 11:02, Conor Dooley wrote:
> > > > 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:
> > > > > > 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]
> >
> > Yeah I understand the conundrum here.
> >
> > > >
> > > > 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...
> > >
> > > Ahem, yeah, I spoke a bit too fast. Looked at the proposal and the
> > > mconfigptr CSR will be accessible by M-mode only so I guess we will have
> > > to find another way...
> >
> > That's not the only problem. Even if you get mconfigptr access, parsing the format
> > is another thing that has to be done. This is early in boot. Although its strictly (pun
> > intended) not a firmware feature extension, I think it's much easier to ask underlying
> > firmware if platform is `Sstrict`. or may be expose something like below
> >
> > `ENABLE_SSTRICT`.
> > Platforms which support `Sstrict` can return success for this while platforms which don't
> > have `Sstrict` can return error code (not supported or not implemented).
> > This way its not feature discovery.
>
> I mean, it's feature discovery in all but name. You're calling it
> enable, but the behaviour you describe is feature discovery - not that I
> am against this being feature discovery since it gets us out of an
> annoying bind.

Yes I know it's cheating but at least for this case, it seems like easy solution which
doesn't break anything. Neither I see it creating any future problems (except FWFT becoming
to look like discovery mechanism :-) and Clement/Atish hating me for that)

Another solution to this could be introducing a riscv config `CONFIG_RISCV_SSTRICT`.
By default always select `CONFIG_RISCV_SSTRICT` and any platform owner who are not
sstrict, they can build their own.
I expect distro (ubuntu, red hat, etc) would want by default `CONFIG_RISCV_SSTRICT`.

> I forget which extension Alex and I discussed previously, but there's
> some mm-related things that you're gonna have to probe super early and
> we need to figure out if we can get that info from ACPI or not. I know
> we discussed it w.r.t. one of the T-Head vendor extensions, but I think
> another standard one got mentioned too.
>
> > It seems like arm64 parses fdt and it reads certain CSRs too
> > (`arch/arm64/kernel/pi/kaslr_early.c`). Although it doesn't look like it has to do any
> > discovery for them.
>
> A decree from the Palmer that we don't support things that do not conform
> in this manner would allow us to do what arm64 does. I brought this up
> originally because it's been discussed before that we cannot rely on
> conformance because we want to support people's platforms, whether they
> comply or not. I'd be wary of making this an exception now, and then
> later on someone makes a platform we want to support that doesn't
> conform and hey presto, we regress KASLR support - even if I think it is
> pretty unlikely that someone is going to repurpose the Zkr CSRs.
>
> One of the problems with only supporting conforming platforms is that
> the definition of conforming changes over time! This has happened with
> the Andes PMU for example, which I believe uses an interrupt number that
> was later co-opted by AIA spec. That conformed at the time, but doesn't
> anymore - do they get to mark themselves as Sstrict?
>
> Maybe we can do this on a case-by-case basis, but it's up to Palmer
> whether or not we can do that.