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

From: Deepak Gupta <debug@rivosinc.com>
To: Conor Dooley <conor@kernel.org>
Cc: "Alexandre Ghiti" <alex@ghiti.fr>,
	"Jesse Taube" <jesse@rivosinc.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev, "Alexandre Ghiti" <alexghiti@rivosinc.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Clément Léger" <cleger@rivosinc.com>,
	"Atish Patra" <atishp@rivosinc.com>
Subject: Re: [PATCH v0] RISC-V: Use Zkr to seed KASLR base address
Date: Fri, 7 Jun 2024 11:51:07 -0700	[thread overview]
Message-ID: <ZmNWmxak9Rc80Kpb@debug.ba.rivosinc.com> (raw)
In-Reply-To: <20240603-stinking-roster-cfad46696ae5@spud>

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

Supervisor could query if Sstrict is implemented and then it can check for
lack of trap on CSR_SEED or straight-away check for presence of Zkr.

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