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

[Ingo Molnar]

* Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> 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().
> Given that the stub's libfdt implementation uses the ordinary, accelerated
> string functions, which rely on hardware handling of unaligned accesses,
> manipulating the FDT with the MMU off may result in alignment faults.
> 
> So fix the situation by moving the update_fdt_memmap() call into the
> callback function invoked by efi_exit_boot_services() right before it
> calls the ExitBootServices() UEFI service (which is arguably a better
> place for it anyway)
> 
> Note that disabling the MMU in ExitBootServices() is not compliant with
> the UEFI spec, and carries great risk due to the fact that switching from
> cached to uncached memory accesses halfway through compiler generated code
> (i.e., involving a stack) can never be done in a way that is architecturally
> safe.
> 
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")
> Tested-by: Riku Voipio <riku.voipio@xxxxxxxxxx>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> ---
>  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;

Nit: could the type cast by dropped?

Thanks,

	Ingo