linux-kernel - Re: [RFC Part1 PATCH 11/13] x86/kernel: validate rom memory before accessing when SEV-SNP is active

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Message-ID: <20210409165302.GF15567@zn.tnic>
Date:   Fri, 9 Apr 2021 18:53:02 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        ak@...ux.intel.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC Part1 PATCH 11/13] x86/kernel: validate rom memory before
 accessing when SEV-SNP is active

On Wed, Mar 24, 2021 at 11:44:22AM -0500, Brijesh Singh wrote:
> +	/*
> +	 * The ROM memory is not part of the E820 system RAM and is not prevalidated by the BIOS.
> +	 * The kernel page table maps the ROM region as encrypted memory, the SEV-SNP requires
> +	 * the all the encrypted memory must be validated before the access.
> +	 */
> +	if (sev_snp_active()) {
> +		unsigned long n, paddr;
> +
> +		n = ((system_rom_resource.end + 1) - video_rom_resource.start) >> PAGE_SHIFT;
> +		paddr = video_rom_resource.start;
> +		early_snp_set_memory_private((unsigned long)__va(paddr), paddr, n);
> +	}

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.

Thx.

-- 
Regards/Gruss,
    Boris.

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