linux-kernel - Re: [RFC PATCH] x86/sev: Disallow userspace access to BIOS region for SEV-SNP guests

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Message-ID: <fd683daa-d953-48ca-8c5d-6f4688ad442c@intel.com>
Date: Tue, 8 Apr 2025 14:19:24 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Tom Lendacky <thomas.lendacky@....com>, Naveen N Rao <naveen@...nel.org>,
 Dan Williams <dan.j.williams@...el.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, linux-coco@...ts.linux.dev,
 Dave Hansen <dave.hansen@...ux.intel.com>, Borislav Petkov <bp@...en8.de>,
 Vishal Annapurve <vannapurve@...gle.com>,
 Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
 Nikolay Borisov <nik.borisov@...e.com>,
 Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [RFC PATCH] x86/sev: Disallow userspace access to BIOS region for
 SEV-SNP guests

On 4/8/25 06:43, Tom Lendacky wrote:
>> Tom/Boris, do you see a problem blocking access to /dev/mem for SEV 
>> guests?
> Not sure why we would suddenly not allow that.

Both TDX and SEV-SNP have issues with allowing access to /dev/mem.
Disallowing access to the individually troublesome regions can fix
_part_ of the problem. But suddenly blocking access is guaranteed to fix
*ALL* the problems forever.

Or, maybe we just start returning 0's for all reads and throw away all
writes. That is probably less likely to break userspace that doesn't
know what it's doing in the first place.