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

Clément Léger
2024-06-10T09:16:45Z

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:
>>>>> 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...

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...

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