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

["Michael S. Tsirkin" via <qemu-devel@nongnu.org>]

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?

> ---
>  hw/i386/x86.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> index 050eedc0c8..fddc20df03 100644
> --- a/hw/i386/x86.c
> +++ b/hw/i386/x86.c
> @@ -1088,8 +1088,15 @@ void x86_load_linux(X86MachineState *x86ms,
>          qemu_guest_getrandom_nofail(setup_data->data, RNG_SEED_LENGTH);
>      }
>  
> -    /* Offset 0x250 is a pointer to the first setup_data link. */
> -    stq_p(header + 0x250, first_setup_data);
> +    /*
> +     * Only modify the header if doing so won't crash EFI boot, which is the
> +     * case only for newer boot protocols, and don't do so either if SEV is
> +     * enabled.
> +     */
> +    if (protocol >= 0x210 && !sev_enabled()) {
> +        /* Offset 0x250 is a pointer to the first setup_data link. */
> +        stq_p(header + 0x250, first_setup_data);
> +    }
>  
>      /*
>       * If we're starting an encrypted VM, it will be OVMF based, which uses the
> -- 
> 2.37.3