Re: [PATCH Part1 v5 29/38] x86/boot: add a pointer to Confidential Computing blob in bootparams

Borislav Petkov
2021-08-27T13:51:00Z

On Fri, Aug 20, 2021 at 10:19:24AM -0500, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@amd.com>
>
> The previously defined Confidential Computing blob is provided to the
> kernel via a setup_data structure or EFI config table entry. Currently
> these are both checked for by boot/compressed kernel to access the
> CPUID table address within it for use with SEV-SNP CPUID enforcement.
>
> To also enable SEV-SNP CPUID enforcement for the run-time kernel,
> similar early access to the CPUID table is needed early on while it's
> still using the identity-mapped page table set up by boot/compressed,
> where global pointers need to be accessed via fixup_pointer().
>
> This is much of an issue for accessing setup_data, and the EFI config
> table helper code currently used in boot/compressed *could* be used in
> this case as well since they both rely on identity-mapping. However, it
> has some reliance on EFI helpers/string constants that would need to be
> accessed via fixup_pointer(), and fixing it up while making it
> shareable between boot/compressed and run-time kernel is fragile and
> introduces a good bit of uglyness.
>
> Instead, this patch adds a boot_params->cc_blob_address pointer that

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

> boot/compressed can initialize so that the run-time kernel can access
> the prelocated CC blob that way instead.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/include/asm/bootparam_utils.h | 1 +
> arch/x86/include/uapi/asm/bootparam.h | 3 ++-
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> 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));
> diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
> index 1ac5acca72ce..bea5cdcdf532 100644
> --- a/arch/x86/include/uapi/asm/bootparam.h
> +++ b/arch/x86/include/uapi/asm/bootparam.h
> @@ -188,7 +188,8 @@ struct boot_params {
> __u32 ext_ramdisk_image; /* 0x0c0 */
> __u32 ext_ramdisk_size; /* 0x0c4 */
> __u32 ext_cmd_line_ptr; /* 0x0c8 */
> - __u8 _pad4[116]; /* 0x0cc */
> + __u8 _pad4[112]; /* 0x0cc */
> + __u32 cc_blob_address; /* 0x13c */

So I know I've heard grub being mentioned in conjunction with this: if
you are ever going to pass this through the boot loader, then you'd need
to update Documentation/x86/zero-page.rst too to state that this field
can be written by the boot loader too.

--
Regards/Gruss,
Boris.