[v2,1/2] efi/libstub: arm64: Force Image reallocation if BSS was not reserved

Distro versions of GRUB replace the usual LoadImage/StartImage calls
used to load the kernel image with some local code that fails to honor
the allocation requirements described in the PE/COFF header, as it
does not account for the image's BSS section at all: it fails to
allocate space for it, and fails to zero initialize it.

Since the EFI stub itself is allocated in the .init segment, which is
in the middle of the image, its BSS section is not impacted by this,
and the main consequence of this omission is that the BSS section may
overlap with memory regions that are already used by the firmware.

So let's warn about this condition, and force image reallocation to
occur in this case, which works around the problem.

Fixes: 82046702e288 ("efi/libstub/arm64: Replace 'preferred' offset with alignment check")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 drivers/firmware/efi/libstub/arm64-stub.c | 49 +++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

On Mon, 2021-07-26 at 12:09 +0200, Ard Biesheuvel wrote:
> Distro versions of GRUB replace the usual LoadImage/StartImage calls
> used to load the kernel image with some local code that fails to
> honor the allocation requirements described in the PE/COFF header, as
> it does not account for the image's BSS section at all: it fails to
> allocate space for it, and fails to zero initialize it.
> 
> Since the EFI stub itself is allocated in the .init segment, which is
> in the middle of the image, its BSS section is not impacted by this,
> and the main consequence of this omission is that the BSS section may
> overlap with memory regions that are already used by the firmware.
> 
> So let's warn about this condition, and force image reallocation to
> occur in this case, which works around the problem.

Ah I did miss that... looking at drivers/firmware/efi/libstub/Makefile:

STUBCOPY_FLAGS-$(CONFIG_ARM64)	+= --prefix-alloc-sections=.init \

and arm64's .lds:

	.init.data : {
		INIT_DATA
		INIT_SETUP(16)
		INIT_CALLS
		CON_INITCALL
		INIT_RAM_FS
		*(.init.rodata.* .init.bss)	/* from the EFI stub */
	}

Gosh ... that does indeed save the day in the relocation case. Any
particular reason why we did this though ? This causes the kernel image
to be bigger than it {c,sh}ould ... We already had cases of broken
bootloaders we knew about or just luck ?

Cheers.
Ben.

> Fixes: 82046702e288 ("efi/libstub/arm64: Replace 'preferred' offset
> with alignment check")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  drivers/firmware/efi/libstub/arm64-stub.c | 49 +++++++++++++++++++-
>  1 file changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c
> b/drivers/firmware/efi/libstub/arm64-stub.c
> index 7bf0a7acae5e..3698c1ce2940 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -34,6 +34,51 @@ efi_status_t check_platform_features(void)
>  	return EFI_SUCCESS;
>  }
>  
> +/*
> + * Distro versions of GRUB may ignore the BSS allocation entirely
> (i.e., fail
> + * to provide space, and fail to zero it). Check for this condition
> by double
> + * checking that the first and the last byte of the image are
> covered by the
> + * same EFI memory map entry.
> + */
> +static bool check_image_region(u64 base, u64 size)
> +{
> +	unsigned long map_size, desc_size, buff_size;
> +	efi_memory_desc_t *memory_map;
> +	struct efi_boot_memmap map;
> +	efi_status_t status;
> +	bool ret = false;
> +	int map_offset;
> +
> +	map.map =	&memory_map;
> +	map.map_size =	&map_size;
> +	map.desc_size =	&desc_size;
> +	map.desc_ver =	NULL;
> +	map.key_ptr =	NULL;
> +	map.buff_size =	&buff_size;
> +
> +	status = efi_get_memory_map(&map);
> +	if (status != EFI_SUCCESS)
> +		return false;
> +
> +	for (map_offset = 0; map_offset < map_size; map_offset +=
> desc_size) {
> +		efi_memory_desc_t *md = (void *)memory_map +
> map_offset;
> +		u64 end = md->phys_addr + md->num_pages *
> EFI_PAGE_SIZE;
> +
> +		/*
> +		 * Find the region that covers base, and return whether
> +		 * it covers base+size bytes.
> +		 */
> +		if (base >= md->phys_addr && base < end) {
> +			ret = (base + size) <= end;
> +			break;
> +		}
> +	}
> +
> +	efi_bs_call(free_pool, memory_map);
> +
> +	return ret;
> +}
> +
>  /*
>   * Although relocatable kernels can fix up the misalignment with
> respect to
>   * MIN_KIMG_ALIGN, the resulting virtual text addresses are subtly
> out of
> @@ -92,7 +137,9 @@ efi_status_t handle_kernel_image(unsigned long
> *image_addr,
>  	}
>  
>  	if (status != EFI_SUCCESS) {
> -		if (IS_ALIGNED((u64)_text, min_kimg_align())) {
> +		if (!check_image_region((u64)_text, kernel_memsize)) {
> +			efi_err("FIRMWARE BUG: Image BSS overlaps
> adjacent EFI memory region\n");
> +		} else if (IS_ALIGNED((u64)_text, min_kimg_align())) {
>  			/*
>  			 * Just execute from wherever we were loaded by
> the
>  			 * UEFI PE/COFF loader if the alignment is
> suitable.