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

[Michael Roth <michael.roth@amd.com>]

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.
> 
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