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

Ingo Molnar
2017-02-01T19:03:48Z

* Ard Biesheuvel <ard.biesheuvel@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().
> 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@vger.kernel.org>
> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")
> Tested-by: Riku Voipio <riku.voipio@linaro.org>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@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;

Nit: could the type cast by dropped?

Thanks,

Ingo