Re: [PATCH v2 5/8] linux/arm: account for COFF headers appearing at unexpected offsets

[Heinrich Schuchardt <xypron.glpk@gmx.de>]

On 10/25/20 2:49 PM, Ard Biesheuvel wrote:
> The way we load the Linux and PE/COFF image headers depends on a fixed
> placement of the COFF header at offset 0x40 into the file. This is a
> reasonable default, given that this is where Linux emits it today.
> However, in order to comply with the PE/COFF spec, which permits this
> header to appear anywhere in the file, let's ensure that we read the
> header from where it actually appears in the file if it is not located
> at offset 0x40.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
> ---
>   grub-core/loader/arm64/linux.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
>
> diff --git a/grub-core/loader/arm64/linux.c b/grub-core/loader/arm64/linux.c
> index 915b6ad7292d..28ff8584a3b5 100644
> --- a/grub-core/loader/arm64/linux.c
> +++ b/grub-core/loader/arm64/linux.c
> @@ -66,6 +66,21 @@ grub_arch_efi_linux_load_image_header (grub_file_t file,
>     grub_dprintf ("linux", "UEFI stub kernel:\n");
>     grub_dprintf ("linux", "PE/COFF header @ %08x\n", lh->hdr_offset);
>
> +  /*
> +   * The PE/COFF spec permits the COFF header to appear anywhere in the file, so
> +   * we need to double check whether it was where we expected it, and if not, we
> +   * must load it from the correct offset into the coff_image_header field of
> +   * struct linux_arch_kernel_header.
> +   */
> +  if ((grub_uint8_t *) lh + lh->hdr_offset != (grub_uint8_t *) &lh->coff_image_header)
> +    {
> +      grub_file_seek (file, lh->hdr_offset);

Isn't this overly complicated? Why don't we first read the whole file
into memory and then analyze it instead of using multiple accesses which
only slows down the process?

Best regards

Heinrich

> +
> +      if (grub_file_read (file, &lh->coff_image_header, sizeof(struct grub_coff_image_header))
> +	  != sizeof(struct grub_coff_image_header))
> +	return grub_error(GRUB_ERR_FILE_READ_ERROR, "failed to read COFF image header");
> +    }
> +
>     return GRUB_ERR_NONE;
>   }
>
>