Re: [PATCH v2] hw/i386: place setup_data at fixed place in memory

[Daniel P . Berrangé]

On Thu, Aug 04, 2022 at 02:44:11AM +0200, Jason A. Donenfeld wrote:
> The boot parameter header refers to setup_data at an absolute address,
> and each setup_data refers to the next setup_data at an absolute address
> too. Currently QEMU simply puts the setup_datas right after the kernel
> image, and since the kernel_image is loaded at prot_addr -- a fixed
> address knowable to QEMU apriori -- the setup_data absolute address
> winds up being just `prot_addr + a_fixed_offset_into_kernel_image`.
> 
> This mostly works fine, so long as the kernel image really is loaded at
> prot_addr. However, OVMF doesn't load the kernel at prot_addr, and
> generally EFI doesn't give a good way of predicting where it's going to
> load the kernel. So when it loads it at some address != prot_addr, the
> absolute addresses in setup_data now point somewhere bogus, causing
> crashes when EFI stub tries to follow the next link.
> 
> Fix this by placing setup_data at some fixed place in memory, relative
> to real_addr, not as part of the kernel image, and then pointing the
> setup_data absolute address to that fixed place in memory. This way,
> even if OVMF or other chains relocate the kernel image, the boot
> parameter still points to the correct absolute address.
> 
> Fixes: 3cbeb52467 ("hw/i386: add device tree support")
> Reported-by: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Richard Henderson <richard.henderson@linaro.org>
> Cc: Peter Maydell <peter.maydell@linaro.org>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: linux-efi@vger.kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  hw/i386/x86.c | 38 ++++++++++++++++++++------------------
>  1 file changed, 20 insertions(+), 18 deletions(-)
> 
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> index 050eedc0c8..8b853abf38 100644
> --- a/hw/i386/x86.c
> +++ b/hw/i386/x86.c


>      if (!legacy_no_rng_seed) {
> -        setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
> -        kernel_size = setup_data_offset + sizeof(struct setup_data) + 
> RNG_SEED_LENGTH;
> -        kernel = g_realloc(kernel, kernel_size);
> -        setup_data = (struct setup_data *)(kernel + setup_data_offset);
> +        setup_data_item_len = sizeof(struct setup_data) + RNG_SEED_LENGTH;
> +        setup_datas = g_realloc(setup_datas, setup_data_total_len + 
> setup_data_item_len);
> +        setup_data = (struct setup_data *)(setup_datas + 
> setup_data_total_len);
>          setup_data->next = cpu_to_le64(first_setup_data);
> -        first_setup_data = prot_addr + setup_data_offset;
> +        first_setup_data = setup_data_base + setup_data_total_len;
> +        setup_data_total_len += setup_data_item_len;
>          setup_data->type = cpu_to_le32(SETUP_RNG_SEED);
>          setup_data->len = cpu_to_le32(RNG_SEED_LENGTH);
>          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);
> +    if (first_setup_data) {
> +            /* Offset 0x250 is a pointer to the first setup_data link. */
> +            stq_p(header + 0x250, first_setup_data);
> +            rom_add_blob("setup_data", setup_datas, setup_data_total_len, 
> setup_data_total_len,
> +                         setup_data_base, NULL, NULL, NULL, NULL, false);
> +    }

The boot measurements with AMD SEV now succeed, but I'm a little
worried about the implications of adding this ROM, when a few lines
later here we're discarding the 'header' changes for AMD SEV. Is
this still going to operate correctly in the guest OS if we've
discarded the header changes below ?

>      /*
>       * If we're starting an encrypted VM, it will be OVMF based, which uses 
> the
>       * efi stub for booting and doesn't require any values to be placed in 
> the
>       * kernel header.  We therefore don't update the header so the hash of 
> the
>       * kernel on the other side of the fw_cfg interface matches the hash of 
> the
>       * file the user passed in.
>       */
>      if (!sev_enabled()) {
>          memcpy(setup, header, MIN(sizeof(header), setup_size));
>      }
>  
>      fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_ADDR, prot_addr);
>      fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_SIZE, kernel_size);
>      fw_cfg_add_bytes(fw_cfg, FW_CFG_KERNEL_DATA, kernel, kernel_size);
>      sev_load_ctx.kernel_data = (char *)kernel;
> -- 
> 2.35.1
> 
> 

With regards,
Daniel
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