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

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

On Fri, Aug 27, 2021 at 03:51:29PM +0200, Borislav Petkov wrote:
> 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.

Right, I think we had discussed this back in v3 or so. But for grub, or
other bootloaders, the idea would be for them to use pass the CC blob
via a struct setup_data corresponding to SETUP_CC_BLOB, introduced in:

  x86/boot: Add Confidential Computing type to setup_data

the boot_params field is only used internally to allow boot/compressed
to hand the CC blob over to kernel proper without kernel proper needing
to rescan for EFI blob (and thus needing all the efi config parsing
stuff).

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
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