[PATCH] efi: fdt: avoid FDT manipulation after ExitBootServices()

[Riku Voipio]

On 1 February 2017 at 15:11, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> Some AArch64 UEFI implementations disable the MMU in ExitBootServices(),
> after which unaligned accesses to RAM are no longer supported.
>
> Commit abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the
> kernel") fixed an issue in the memory map handling of the stub FDT code,
> but inadvertently created an issue with such firmwares, by moving some
> of the FDT manipulation to after the invocation of ExitBootServices().
> However, the stub's libfdt implementation uses the ordinary, accelerated
> string functions, which rely on hardware handling of unaligned accesses,
> resulting in alignment faults when executed with the MMU off.
>
> Whether disabling the MMU in ExitBootServices() complies with the UEFI
> spec is unclear, but it is a reality we have to deal with, given that
> it wasn't a problem before commit abfb7b686a3e was applied.
>
> So fix the situation by moving the update_fdt_memmap() into the callback
> invoked by efi_exit_boot_services() right before it hits the button.
>
> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

Tested-by: Riku Voipio <riku.voipio at linaro.org>

> ---
>  drivers/firmware/efi/libstub/fdt.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/fdt.c b/drivers/firmware/efi/libstub/fdt.c
> index 921dfa047202..260c4b4b492e 100644
> --- a/drivers/firmware/efi/libstub/fdt.c
> +++ b/drivers/firmware/efi/libstub/fdt.c
> @@ -187,6 +187,7 @@ static efi_status_t update_fdt_memmap(void *fdt, struct efi_boot_memmap *map)
>  struct exit_boot_struct {
>         efi_memory_desc_t *runtime_map;
>         int *runtime_entry_count;
> +       void *new_fdt_addr;
>  };
>
>  static efi_status_t exit_boot_func(efi_system_table_t *sys_table_arg,
> @@ -202,7 +203,7 @@ static efi_status_t exit_boot_func(efi_system_table_t *sys_table_arg,
>         efi_get_virtmap(*map->map, *map->map_size, *map->desc_size,
>                         p->runtime_map, p->runtime_entry_count);
>
> -       return EFI_SUCCESS;
> +       return update_fdt_memmap(p->new_fdt_addr, map);
>  }
>
>  /*
> @@ -300,22 +301,13 @@ efi_status_t allocate_new_fdt_and_exit_boot(efi_system_table_t *sys_table,
>
>         priv.runtime_map = runtime_map;
>         priv.runtime_entry_count = &runtime_entry_count;
> +       priv.new_fdt_addr = (void *)*new_fdt_addr;
>         status = efi_exit_boot_services(sys_table, handle, &map, &priv,
>                                         exit_boot_func);
>
>         if (status == EFI_SUCCESS) {
>                 efi_set_virtual_address_map_t *svam;
>
> -               status = update_fdt_memmap((void *)*new_fdt_addr, &map);
> -               if (status != EFI_SUCCESS) {
> -                       /*
> -                        * The kernel won't get far without the memory map, but
> -                        * may still be able to print something meaningful so
> -                        * return success here.
> -                        */
> -                       return EFI_SUCCESS;
> -               }
> -
>                 /* Install the new virtual address map */
>                 svam = sys_table->runtime->set_virtual_address_map;
>                 status = svam(runtime_entry_count * desc_size, desc_size,
> --
> 2.7.4
>