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 e3df82d5d37a83f2fe6958ab5ecfabe8581e286a..70be9c87fb673234899457bb7eda9fa09213e657 100644 (file)
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -590,7 +590,7 @@ 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 3bac68fb7ff1c67db25f66a29908b0c09a9370f6..7feb70d32d95570dd808b990ae78110b646e0c3e 100644 (file)
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1161,7 +1161,7 @@ void efi_retrieve_tpm2_eventlog(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;