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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 19 May 2023 00:33:20 +0200
Message-ID: <CAMj1kXFvfyPibZWTLUaLb6pJQWJbV2p7wV__dcCAyjLAuPrtQg@mail.gmail.com>
Subject: Re: [PATCH v2 18/20] x86: efistub: Avoid legacy decompressor when
 doing EFI boot
To:     Tom Lendacky <thomas.lendacky@amd.com>
Cc:     linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Evgeniy Baskov <baskov@ispras.ru>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexey Khoroshilov <khoroshilov@ispras.ru>,
        Peter Jones <pjones@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Dave Young <dyoung@redhat.com>,
        Mario Limonciello <mario.limonciello@amd.com>,
        Kees Cook <keescook@chromium.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Thu, 18 May 2023 at 22:48, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>
> On 5/8/23 02:03, Ard Biesheuvel wrote:
> > The bare metal decompressor code was never really intended to run in a
> > hosted environment such as the EFI boot services, and does a few things
> > that are problematic in the context of EFI boot now that the logo
> > requirements are getting tighter.
> >
> > In particular, the decompressor moves its own executable image around in
> > memory, and relies on demand paging to populate the identity mappings,
> > and these things are difficult to support in a context where memory is
> > not permitted to be mapped writable and executable at the same time or,
> > at the very least, is mapped non-executable by default, and needs
> > special treatment for this restriction to be lifted.
> >
> > Since EFI already maps all of memory 1:1, we don't need to create new
> > page tables or handle page faults when decompressing the kernel. That
> > means there is also no need to replace the special exception handlers
> > for SEV. Generally, there is little need to do anything that the
> > decompressor does beyond
> >
> > - initialize SEV encryption, if needed,
> > - perform the 4/5 level paging switch, if needed,
> > - decompress the kernel
> > - relocate the kernel
> >
> > So let's do all of this from the EFI stub code, and avoid the bare metal
> > decompressor altogether.
>
> This patch crashes SEV guests, probably because of the BSS is accessed
> encrypted and results in ciphertext for what would be a zero for a non-SEV
> guest. After pushing patch #19 everything started working again. From a
> bisectability perspective, you probably want patch #19 before this one.
>

Noted, thanks.