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

From: Borislav Petkov <bp@alien8.de>
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	kvm@vger.kernel.org, ak@linux.intel.com,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Tony Luck <tony.luck@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	David Rientjes <rientjes@google.com>,
	Sean Christopherson <seanjc@google.com>
Subject: Re: [RFC Part1 PATCH 11/13] x86/kernel: validate rom memory before accessing when SEV-SNP is active
Date: Fri, 9 Apr 2021 18:53:02 +0200	[thread overview]
Message-ID: <20210409165302.GF15567@zn.tnic> (raw)
In-Reply-To: <20210324164424.28124-12-brijesh.singh@amd.com>

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