Re: [PATCH Part1 RFC v3 20/22] x86/boot: Add Confidential Computing address to setup_header

["Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@linux.intel.com>]

+ Andi, Kirill (For TDX specific review)

On 6/23/21 8:19 PM, Michael Roth wrote:
> On Wed, Jun 23, 2021 at 12:22:23PM +0200, Borislav Petkov wrote:
>> On Tue, Jun 22, 2021 at 11:30:43PM -0500, Michael Roth wrote:
>>> Quoting Borislav Petkov (2021-06-18 10:05:28)
>>>> On Fri, Jun 18, 2021 at 08:57:12AM -0500, Brijesh Singh wrote:
>>>>> Don't have any strong reason to keep it separate, I can define a new
>>>>> type and use the setup_data to pass this information.
>>>>
>>>> setup_data is exactly for use cases like that - pass a bunch of data
>>>> to the kernel. So there's no need for a separate thing. Also see that
>>>> kernel_info thing which got added recently for read_only data.
>>>
>>> Hi Boris,
>>>
>>> There's one side-effect to this change WRT the CPUID page (which I think
>>> we're hoping to include in RFC v4).
>>>
>>> With CPUID page we need to access it very early in boot, for both
>>> boot/compressed kernel, and the uncompressed kernel. At first this was
>>> implemented by moving the early EFI table parsing code from
>>> arch/x86/kernel/boot/compressed/acpi.c into a little library to handle early
>>> EFI table parsing to fetch the Confidential Computing blob to get the CPUID
>>> page address.
>>>
>>> This was a bit messy since we needed to share that library between
>>> boot/compressed and uncompressed, and at that early stage things like
>>> fixup_pointer() are needed in some places, else even basic things like
>>> accessing EFI64_LOADER_SIGNATURE and various EFI helper functions could crash
>>> in uncompressed otherwise, so the library code needed to be fixed up
>>> accordingly.
>>>
>>> To simplify things we ended up simply keeping the early EFI table parsing in
>>> boot/compressed, and then having boot/compressed initialize
>>> setup_data.cc_blob_address so that the uncompressed kernel could access it
>>> from there (acpi does something similar with rdsp address).
>>
>> Yes, except the rsdp address is not vendor-specific but an x86 ACPI
>> thing, so pretty much omnipresent.
>>
>> Also, acpi_rsdp_addr is part of boot_params and that struct is full
>> of padding holes and obsolete members so reusing a u32 there is a lot
>> "easier" than changing the setup_header. So can you put that address in
>> boot_params instead?
> 
> Thanks for the suggestion! I tried something like the below and that seems to
> work pretty well. I'm not sure if that's the best spot or not though, it
> seems like it might be a good idea to leave some padding after eddbuf in
> case it needs to grow in the future. I'll look into that a bit more.
> 
> One downside to this is we still need something in the boot protocol,
> either via setup_data, or setup_header directly. Having it in
> setup_header avoids the need to also have to add a field to boot_params
> for the boot/compressed->uncompressed passing, but maybe that's not a good
> enough justification. Perhaps if the TDX folks have similar needs though.
> 
> diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
> index 1ac5acca72ce..0824c8646861 100644
> --- a/arch/x86/include/uapi/asm/bootparam.h
> +++ b/arch/x86/include/uapi/asm/bootparam.h
> @@ -218,7 +218,8 @@ struct boot_params {
>          struct boot_e820_entry e820_table[E820_MAX_ENTRIES_ZEROPAGE]; /* 0x2d0 */
>          __u8  _pad8[48];                                /* 0xcd0 */
>          struct edd_info eddbuf[EDDMAXNR];               /* 0xd00 */
> -       __u8  _pad9[276];                               /* 0xeec */
> +       __u32 cc_blob_address;							/* 0xeec */
> +       __u8  _pad9[272];                               /* 0xef0 */
>   } __attribute__((packed));
> 
> diff --git a/arch/x86/include/asm/bootparam_utils.h b/arch/x86/include/asm/bootparam_utils.h
> index 981fe923a59f..53e9b0620d96 100644
> --- a/arch/x86/include/asm/bootparam_utils.h
> +++ b/arch/x86/include/asm/bootparam_utils.h
> @@ -74,6 +74,7 @@ static void sanitize_boot_params(struct boot_params *boot_params)
>                          BOOT_PARAM_PRESERVE(hdr),
>                          BOOT_PARAM_PRESERVE(e820_table),
>                          BOOT_PARAM_PRESERVE(eddbuf),
> +                       BOOT_PARAM_PRESERVE(cc_blob_address),
>                  };
> 
>                  memset(&scratch, 0, sizeof(scratch));
> 
>>
>>> Now that we're moving it to setup_data, this becomes a bit more awkward,
>>> since we need to reserve memory in boot/compressed to store the setup_data
>>> entry, then add it to the linked list to pass along to uncompressed kernel.
>>> In turn that also means we need to add an identity mapping for this in
>>> ident_map_64.c, so I'm not sure that's the best approach.
>>>
>>> So just trying to pin what the best approach is:
>>>
>>> a) move cc_blob to setup_data, and do the above-described to pass
>>>     cc_blob_address from boot/compressed to uncompressed to avoid early
>>>     EFI parsing in uncompressed
>>> b) move cc_blob to setup_data, and do the EFI table parsing in both
>>>     boot/compressed. leave setup_data allocation/init for BIOS/bootloader
>>> c) keep storing cc_blob_address in setup_header.cc_blob_address
>>> d) something else?
>>
>> Leaving in the whole text for newly CCed TDX folks in case they're going
>> to need something like that.
>>
>> And if so, then both vendors should even share the field definition.
>>
>> Dave, Sathya, you can find the whole subthread here:
>>
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20210602140416.23573-21-brijesh.singh%40amd.com&data=04%7C01%7Cmichael.roth%40amd.com%7C3b352c4b944c4d95bbdb08d93630d0eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637600405622460196%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=464O7JxibsbjC3bc0LkGcztdb4kCYH7kcQAcqohJhug%3D&reserved=0
>>
>> in case you need background info on the topic at hand.
>>
>> Thx.
>>
>> -- 
>> Regards/Gruss,
>>      Boris.
>>
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpeople.kernel.org%2Ftglx%2Fnotes-about-netiquette&data=04%7C01%7Cmichael.roth%40amd.com%7C3b352c4b944c4d95bbdb08d93630d0eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637600405622460196%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=ruCM7CNgPCNPkrOoiNts1ZKi5k7JSUumln7qQMP%2BMi0%3D&reserved=0

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer