linux-kernel - Re: [PATCH v3 00/17] x86: head

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Date:   Wed, 23 Nov 2022 11:49:44 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michael Roth <michael.roth@....com>
Subject: Re: [PATCH v3 00/17] x86: head_64.S spring cleaning

On Tue, Nov 22, 2022 at 03:49:29PM -0600, Tom Lendacky wrote:
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index cb5f0befee57..a0bfd31358ba 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -23,7 +23,7 @@
>  const efi_system_table_t *efi_system_table;
>  const efi_dxe_services_table_t *efi_dxe_table;
> -u32 image_offset;
> +u32 image_offset __section(".data");
>  static efi_loaded_image_t *image = NULL;
>  static efi_status_t
> 
> I assume it has to do with being in .data vs .bss and not being explicitly
> cleared with the encryption bit set. With the change to put image_offset in
> the .data section, it is read as zero, where as when it was in the .bss
> section it was reading "ciphertext".

Hmm, two points about this:

1. Can we do

u32 image_offset __bss_decrypted;

here instead? We have this special section just for that fun and it
self-documents this way.

2. Also, why does my SEV-ES guest boot just fine without that change?

[    0.000000] Linux version 6.1.0-rc6+ (root@ml) (gcc (Debian 11.3.0-1) 11.3.0, GNU ld (GNU Binutils for Debian) 2.38) #1 SMP PREEMPT_DYNAMIC Wed Nov 23 11:27:17 CET 2022
...
[    0.336132] Memory Encryption Features active: AMD SEV SEV-ES

Thx.

-- 
Regards/Gruss,
    Boris.

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