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rollback: Include board-generated entropy when adding entropy

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Mix in board-generated entropy with the externally provided one,
which should help make the per-device secret stronger.

BRANCH=none
BUG=b:38486828
TEST=reboot; rollbackaddent Hello => works fine when USB is connected,
     fails otherwise, as board-generated entropy relies on USB timing.

Change-Id: I314f44759c5f8b859913a748db95e9d42b5cdd11
Reviewed-on: https://chromium-review.googlesource.com/518609
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
This commit is contained in:
Nicolas Boichat
2017-05-31 12:41:01 +08:00
committed by chrome-bot
parent 39db721f9a
commit 6d05a31a44
3 changed files with 24 additions and 4 deletions
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1
board/hammer/board.h
View File

@@ -196,6 +196,7 @@
*/
#define CONFIG_ROLLBACK
#define CONFIG_ROLLBACK_SECRET_SIZE 32
#define CONFIG_ROLLBACK_SECRET_LOCAL_ENTROPY_SIZE 32
#define CONFIG_FLASH_PROTECT_RW
#ifdef SECTION_IS_RW
#undef CONFIG_ROLLBACK_UPDATE

20
common/rollback.c
View File

@@ -135,24 +135,34 @@ int rollback_lock(void)
#ifdef CONFIG_ROLLBACK_UPDATE
#ifdef CONFIG_ROLLBACK_SECRET_SIZE
static void add_entropy(uint8_t *dst, const uint8_t *src,
static int add_entropy(uint8_t *dst, const uint8_t *src,
uint8_t *add, unsigned int add_len)
{
#ifdef CONFIG_SHA256
BUILD_ASSERT(SHA256_DIGEST_SIZE == CONFIG_ROLLBACK_SECRET_SIZE);
struct sha256_ctx ctx;
uint8_t *hash;
uint8_t extra;
int i;
SHA256_init(&ctx);
SHA256_update(&ctx, src, CONFIG_ROLLBACK_SECRET_SIZE);
SHA256_update(&ctx, add, add_len);
/* TODO(b:38486828): Add other sources of entropy (e.g. device id) */
#ifdef CONFIG_ROLLBACK_SECRET_LOCAL_ENTROPY_SIZE
/* Add some locally produced entropy */
for (i = 0; i < CONFIG_ROLLBACK_SECRET_LOCAL_ENTROPY_SIZE; i++) {
if (!board_get_entropy(&extra, 1))
return 0;
SHA256_update(&ctx, &extra, 1);
}
#endif
hash = SHA256_final(&ctx);
memcpy(dst, hash, CONFIG_ROLLBACK_SECRET_SIZE);
#else
#error "Adding entropy to secret in rollback region requires SHA256."
#endif
return 1;
}
#endif /* CONFIG_ROLLBACK_SECRET_SIZE */
@@ -212,8 +222,10 @@ static int rollback_update(int32_t next_min_version,
* If we are provided with some entropy, add it to secret. Otherwise,
* data.secret is left untouched and written back to the other region.
*/
if (entropy)
add_entropy(data.secret, data.secret, entropy, length);
if (entropy) {
if (!add_entropy(data.secret, data.secret, entropy, length))
return EC_ERROR_UNCHANGED;
}
#endif
data.cookie = CROS_EC_ROLLBACK_COOKIE;

7
include/config.h
View File

@@ -1159,6 +1159,13 @@
/* If defined, add support for storing some entropy in the rollback region. */
#undef CONFIG_ROLLBACK_SECRET_SIZE
/*
* If defined, inject some locally generated entropy when secret is updated,
* using board_get_entropy function.
* Large values may take a long time to generate.
*/
#undef CONFIG_ROLLBACK_SECRET_LOCAL_ENTROPY_SIZE
/* If defined, we can update rollback information (RW can unset this). */
#define CONFIG_ROLLBACK_UPDATE