Re: [PATCH RFC v1] hw/i386: place setup_data at fixed place in memory

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 03, 2022 at 07:02:35PM +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, 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.
> 
> === NOTE NOTE NOTE NOTE NOTE ===
> This commit is currently garbage! It fixes the boot test case, but it
> just picks the address 0x10000000. That's probably not a good idea. If
> somebody with some x86 architectural knowledge could let me know a
> better reserved place to put this, that'd be very appreciated.
> 
> 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, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/i386/x86.c b/hw/i386/x86.c
> index 050eedc0c8..0b0083b345 100644
> --- a/hw/i386/x86.c
> +++ b/hw/i386/x86.c
> @@ -773,9 +773,9 @@ void x86_load_linux(X86MachineState *x86ms,
>      bool linuxboot_dma_enabled = X86_MACHINE_GET_CLASS(x86ms)->fwcfg_dma_enabled;
>      uint16_t protocol;
>      int setup_size, kernel_size, cmdline_size;
> -    int dtb_size, setup_data_offset;
> +    int dtb_size, setup_data_item_len, setup_data_total_len = 0;
>      uint32_t initrd_max;
> -    uint8_t header[8192], *setup, *kernel;
> +    uint8_t header[8192], *setup, *kernel, *setup_datas = NULL;
>      hwaddr real_addr, prot_addr, cmdline_addr, initrd_addr = 0, first_setup_data = 0;
>      FILE *f;
>      char *vmode;
> @@ -1048,6 +1048,8 @@ void x86_load_linux(X86MachineState *x86ms,
>      }
>      fclose(f);
>  
> +#define SETUP_DATA_PHYS_BASE 0x10000000
> +
>      /* append dtb to kernel */
>      if (dtb_filename) {
>          if (protocol < 0x209) {
> @@ -1062,34 +1064,36 @@ void x86_load_linux(X86MachineState *x86ms,
>              exit(1);
>          }
>  
> -        setup_data_offset = QEMU_ALIGN_UP(kernel_size, 16);
> -        kernel_size = setup_data_offset + sizeof(struct setup_data) + dtb_size;
> -        kernel = g_realloc(kernel, kernel_size);
> -
> -
> -        setup_data = (struct setup_data *)(kernel + setup_data_offset);
> +        setup_data_item_len = sizeof(struct setup_data) + dtb_size;
> +        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_PHYS_BASE + setup_data_total_len;
> +        setup_data_total_len += setup_data_item_len;
>          setup_data->type = cpu_to_le32(SETUP_DTB);
>          setup_data->len = cpu_to_le32(dtb_size);
> -
>          load_image_size(dtb_filename, setup_data->data, dtb_size);
>      }
>  
>      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) + SETUP_RNG_SEED;
> +        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_PHYS_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_PHYS_BASE, NULL, NULL, NULL, NULL, false);
> +    }
> +
>

Allocating memory on x86 is tricky business.  Can we maybe use bios-linker-loader
with COMMAND_WRITE_POINTER to get an address from firmware?

  
>      /*
>       * If we're starting an encrypted VM, it will be OVMF based, which uses the
> -- 
> 2.35.1