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

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Message-ID: <YSjt4YDQR8vDeOdI@zn.tnic>
Date:   Fri, 27 Aug 2021 15:51:29 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 29/38] x86/boot: add a pointer to Confidential
 Computing blob in bootparams

On Fri, Aug 20, 2021 at 10:19:24AM -0500, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@....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@....com>
> Signed-off-by: Brijesh Singh <brijesh.singh@....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.

https://people.kernel.org/tglx/notes-about-netiquette