RE: [PATCH] efi/libstub/arm64: avoid image_base value from efi_loaded_image

[Michael Kelley <mikelley@microsoft.com>]

From: Ard Biesheuvel <ardb@kernel.org> Sent: Saturday, March 28, 2020 1:58 PM
> 
> Commit 9f9223778ef3 ("efi/libstub/arm: Make efi_entry() an ordinary
> PE/COFF entrypoint") did some code refactoring to get rid of the
> EFI entry point assembler code, and in the process, it got rid of the
> assignment of image_addr to the value of _text. Instead, it switched
> to using the image_base field of the efi_loaded_image struct provided
> by UEFI, which should contain the same value.
> 
> However, Michael reports that this is not the case: older GRUB builds
> corrupt this value in some way, and since we can easily switch back to
> referring to _text to discover this value, let's simply do that.
> 
> While at it, fix another issue in commit 9f9223778ef3, which may result
> in the unassigned image_addr to be misidentified as the preferred load
> offset of the kernel, which is unlikely but will cause a boot crash if
> it does occur.
> 
> Finally, let's add a warning if the _text vs. image_base discrepancy is
> detected, so we can tell more easily how widespread this issue actually
> is.
> 
> Reported-by: Michael Kelley <mikelley@microsoft.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  drivers/firmware/efi/libstub/arm64-stub.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

Tested in a Hyper-V VM.  With the 2.02~beta2 version of grub,
the FIRMWARE BUG message is output and then Linux correctly
boots.  With the 2.04-4 version of grub, Linux correctly boots with
no error messages.

Tested-by: Michael Kelley <mikelley@microsoft.com>

> 
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c
> b/drivers/firmware/efi/libstub/arm64-stub.c
> index 9254cd8ab2d3..db0c1a9c1699 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -116,6 +116,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>  		 * Mustang), we can still place the kernel at the address
>  		 * 'dram_base + TEXT_OFFSET'.
>  		 */
> +		*image_addr = (unsigned long)_text;
>  		if (*image_addr == preferred_offset)
>  			return EFI_SUCCESS;
> 
> @@ -140,7 +141,11 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
>  		}
>  		*image_addr = *reserve_addr + TEXT_OFFSET;
>  	}
> -	memcpy((void *)*image_addr, image->image_base, kernel_size);
> +
> +	if (image->image_base != _text)
> +		pr_efi_err("FIRMWARE BUG: efi_loaded_image_t::image_base has bogus
> value\n");
> +
> +	memcpy((void *)*image_addr, _text, kernel_size);
> 
>  	return EFI_SUCCESS;
>  }
> --
> 2.17.1