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Re: [PATCH v2 1/2] x86: only modify setup_data if the boot protocol indi


From: Ard Biesheuvel
Subject: Re: [PATCH v2 1/2] x86: only modify setup_data if the boot protocol indicates safety
Date: Tue, 6 Sep 2022 13:14:50 +0200

(cc Laszlo)

On Tue, 6 Sept 2022 at 12:45, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Sep 06, 2022 at 12:43:55PM +0200, Jason A. Donenfeld wrote:
> > On Tue, Sep 6, 2022 at 12:40 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Tue, Sep 06, 2022 at 12:36:56PM +0200, Jason A. Donenfeld wrote:
> > > > It's only safe to modify the setup_data pointer on newer kernels where
> > > > the EFI stub loader will ignore it. So condition setting that offset on
> > > > the newer boot protocol version. While we're at it, gate this on SEV 
> > > > too.
> > > > This depends on the kernel commit linked below going upstream.
> > > >
> > > > Cc: Gerd Hoffmann <kraxel@redhat.com>
> > > > Cc: Laurent Vivier <laurent@vivier.eu>
> > > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > > Cc: Peter Maydell <peter.maydell@linaro.org>
> > > > Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
> > > > Cc: Richard Henderson <richard.henderson@linaro.org>
> > > > Cc: Ard Biesheuvel <ardb@kernel.org>
> > > > Link: 
> > > > https://lore.kernel.org/linux-efi/20220904165321.1140894-1-Jason@zx2c4.com/
> > > > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> > >
> > > BTW what does it have to do with SEV?
> > > Is this because SEV is not going to trust the data to be random anyway?
> >
> > Daniel (now CC'd) pointed out in one of the previous threads that this
> > breaks SEV, because the image hash changes.
> >
> > Jason
>
> Oh I see. I'd add a comment maybe and definitely mention this
> in the commit log.
>

This does raise the question (as I mentioned before) how things like
secure boot and measured boot are affected when combined with direct
kernel boot: AIUI, libvirt uses direct kernel boot at guest
installation time, and modifying setup_data will corrupt the image
signature.



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