From: Ard Biesheuvel <ardb@xxxxxxxxxx>
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. [stable:
READ_ONCE() wasn't backported in the first place.]
Cc: <stable@xxxxxxxxxxxxxxx> # v4.14+
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Reviewed-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Acked-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
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 7098744f9276..f0ef2643b70e 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -556,7 +556,7 @@ 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 ec89e8bcc92f..789a194e18a4 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1655,7 +1655,7 @@ 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;
--
2.38.1
