efi: random: reduce seed size to 32 bytes

commit 161a438d730dade2ba2b1bf8785f0759aba4ca5f upstream.

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>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 24365601fbbf9b1792eb775be3209d1935d9d5cb..ed981f5e29ae9baf91f1ac8651b7f9857c9aa86b 100644 (file)
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -532,7 +532,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
 
                seed = early_memremap(efi.rng_seed, sizeof(*seed));
                if (seed != NULL) {
-                       size = 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 2c63afd68978a968b8a6ba04a1dc8f80e4cf4e51..8b25cd2bed40c5c72e1e89042cc92f8d1f0909f0 100644 (file)
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1607,7 +1607,7 @@ efi_status_t efi_exit_boot_services(efi_system_table_t *sys_table,
                                    void *priv,
                                    efi_exit_boot_map_processing priv_func);
 
-#define EFI_RANDOM_SEED_SIZE           64U
+#define EFI_RANDOM_SEED_SIZE           32U // BLAKE2S_HASH_SIZE
 
 struct linux_efi_random_seed {
        u32     size;