linux-kernel - Re: [PATCH Part1 RFC v2 16/20] x86/kernel: Validate rom memory before accessing when SEV-SNP is active

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Date:   Thu, 27 May 2021 13:49:05 +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,
        tglx@...utronix.de, jroedel@...e.de, thomas.lendacky@....com,
        pbonzini@...hat.com, mingo@...hat.com, dave.hansen@...el.com,
        rientjes@...gle.com, seanjc@...gle.com, peterz@...radead.org,
        hpa@...or.com, tony.luck@...el.com
Subject: Re: [PATCH Part1 RFC v2 16/20] x86/kernel: Validate rom memory
 before accessing when SEV-SNP is active

On Fri, Apr 30, 2021 at 07:16:12AM -0500, Brijesh Singh wrote:
> +	/*
> +	 * The ROM memory is not part of the E820 system RAM and is not pre-validated
> +	 * by the BIOS. The kernel page table maps the ROM region as encrypted memory,
> +	 * the SEV-SNP requires the encrypted memory must be validated before the
> +	 * access. Validate the ROM before accessing it.
> +	 */
> +	n = ((system_rom_resource.end + 1) - video_rom_resource.start) >> PAGE_SHIFT;
> +	early_snp_set_memory_private((unsigned long)__va(video_rom_resource.start),
> +			video_rom_resource.start, n);

>From last review:

I don't like this sprinkling of SNP-special stuff that needs to be done,
around the tree. Instead, pls define a function called

        snp_prep_memory(unsigned long pa, unsigned int num_pages, enum operation);

or so which does all the manipulation needed and the callsites only
simply unconditionally call that function so that all detail is
extracted and optimized away when not config-enabled.

-- 
Regards/Gruss,
    Boris.

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