Re: [PATCH v3 1/3] efi: random: reduce seed size to 32 bytes

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

On Thu, Oct 20, 2022 at 10:39:08AM +0200, Ard Biesheuvel wrote:
> We no longer need at least 64 bytes of random seed to permit the early
> crng init to complete. The RNG is now based on Blake2s, so reduce the
> EFI seed size to the Blake2s hash size, which is sufficient for our
> purposes.
> 
> While at it, drop the READ_ONCE(), which was supposed to prevent size
> from being evaluated after seed was unmapped. However, this cannot
> actually happen, so READ_ONCE() is unnecessary here.
> 
> Cc: <stable@vger.kernel.org> # v4.14+
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  drivers/firmware/efi/efi.c | 2 +-
>  include/linux/efi.h        | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 9624735f1575..a949509de62f 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -609,7 +609,7 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
>  
>  		seed = early_memremap(efi_rng_seed, sizeof(*seed));
>  		if (seed != NULL) {
> -			size = READ_ONCE(seed->size);
> +			size = min(seed->size, EFI_RANDOM_SEED_SIZE);
>  			early_memunmap(seed, sizeof(*seed));
>  		} else {
>  			pr_err("Could not map UEFI random seed!\n");
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index da3974bf05d3..cf96f8d5f15f 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -1225,7 +1225,7 @@ efi_status_t efi_random_get_seed(void);
>  	arch_efi_call_virt_teardown();					\
>  })
>  
> -#define EFI_RANDOM_SEED_SIZE		64U
> +#define EFI_RANDOM_SEED_SIZE		32U // BLAKE2S_HASH_SIZE
>  
>  struct linux_efi_random_seed {
>  	u32	size;
> -- 
> 2.35.1
> 

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>