[U-Boot] [PATCH v2 4/4] autoboot.c: Add feature to stop autobooting via SHA256 encrypted password

Simon Glass
2015-05-10T14:49:08Z

Hi Stefan,

On 8 May 2015 at 01:52, Stefan Roese <sr@denx.de> wrote:
> This patch adds the feature to only stop the autobooting, and therefor
> boot into the U-Boot prompt, when the input string / password matches
> a values that is encypted via a SHA256 hash and saved in the environment.
>
> This feature is enabled by defined these config options:
> CONFIG_AUTOBOOT_KEYED
> CONFIG_AUTOBOOT_STOP_STR_SHA256
>
> Signed-off-by: Stefan Roese <sr@denx.de>
> Cc: Simon Glass <sjg@chromium.org>
> Cc: Magnus Lilja <lilja.magnus@gmail.com>
> ---
> v2:
> - AUTOBOOT_STOP_STR_SHA256 is a string and not bool
> - Add input key length check as suggested by Magnus
> - Add "constant-length" time compare function as suggested
> by Magnus
>
> common/Kconfig | 20 +++++++--
> common/autoboot.c | 132 +++++++++++++++++++++++++++++++++++++++++++++---------
> 2 files changed, 128 insertions(+), 24 deletions(-)

Reviewed-by: Simon Glass <sjg@chromium.org>

A few optional ideas below.

>
> diff --git a/common/Kconfig b/common/Kconfig
> index ff150f0..71c5c10 100644
> --- a/common/Kconfig
> +++ b/common/Kconfig
> @@ -45,9 +45,14 @@ config AUTOBOOT_PROMPT
> the responsibility of the user to select only such arguments
> that are valid in the given context.
>
> +config AUTOBOOT_ENCRYPTION
> + bool "Enable encryption in autoboot stopping"
> + depends on AUTOBOOT_KEYED
> + default n
> +
> config AUTOBOOT_DELAY_STR
> string "Delay autobooting via specific input key / string"
> - depends on AUTOBOOT_KEYED
> + depends on AUTOBOOT_KEYED && !AUTOBOOT_ENCRYPTION
> help
> This option delays the automatic boot feature by issuing
> a specific input key or string. If CONFIG_AUTOBOOT_DELAY_STR
> @@ -59,7 +64,7 @@ config AUTOBOOT_DELAY_STR
>
> config AUTOBOOT_STOP_STR
> string "Stop autobooting via specific input key / string"
> - depends on AUTOBOOT_KEYED
> + depends on AUTOBOOT_KEYED && !AUTOBOOT_ENCRYPTION
> help
> This option enables stopping (aborting) of the automatic
> boot feature only by issuing a specific input key or
> @@ -71,7 +76,7 @@ config AUTOBOOT_STOP_STR
>
> config AUTOBOOT_KEYED_CTRLC
> bool "Enable Ctrl-C autoboot interruption"
> - depends on AUTOBOOT_KEYED
> + depends on AUTOBOOT_KEYED && !AUTOBOOT_ENCRYPTION
> default n
> help
> This option allows for the boot sequence to be interrupted
> @@ -79,6 +84,15 @@ config AUTOBOOT_KEYED_CTRLC
> Setting this variable provides an escape sequence from the
> limited "password" strings.
>
> +config AUTOBOOT_STOP_STR_SHA256
> + string "Stop autobooting via SHA256 encrypted password"
> + depends on AUTOBOOT_KEYED && AUTOBOOT_ENCRYPTION
> + help
> + This option adds the feature to only stop the autobooting,
> + and therefore boot into the U-Boot prompt, when the input
> + string / password matches a values that is encypted via
> + a SHA256 hash and saved in the environment.
> +
> endmenu
>
> comment "Commands"
> diff --git a/common/autoboot.c b/common/autoboot.c
> index f72eb18..dde3d04 100644
> --- a/common/autoboot.c
> +++ b/common/autoboot.c
> @@ -12,6 +12,7 @@
> #include <fdtdec.h>
> #include <menu.h>
> #include <post.h>
> +#include <u-boot/sha256.h>
>
> DECLARE_GLOBAL_DATA_PTR;
>
> @@ -26,15 +27,80 @@ DECLARE_GLOBAL_DATA_PTR;
> /* Stored value of bootdelay, used by autoboot_command() */
> static int stored_bootdelay;
>
> -/***************************************************************************
> - * Watch for 'delay' seconds for autoboot stop or autoboot delay string.
> - * returns: 0 - no key string, allow autoboot 1 - got key string, abort
> +#if defined(CONFIG_AUTOBOOT_KEYED)
> +#if defined(CONFIG_AUTOBOOT_STOP_STR_SHA256)
> +
> +/*
> + * Use a "constant-length" time compare function for this
> + * hash compare:
> + *
> + * https://crackstation.net/hashing-security.htm
> */
> -# if defined(CONFIG_AUTOBOOT_KEYED)
> -static int abortboot_keyed(int bootdelay)
> +static int slow_equals(u8 *a, u8 *b, int len)
> +{
> + int diff = 0;
> + int i;
> +
> + for (i = 0; i < len; i++)
> + diff |= a[i] ^ b[i];
> +
> + return diff == 0;
> +}
> +
> +static int passwd_abort(uint64_t etime)
> +{
> + const char *sha_env_str = getenv("bootstopkeysha256");
> + u8 sha_env[SHA256_SUM_LEN];
> + u8 sha[SHA256_SUM_LEN];
> + char presskey[MAX_DELAY_STOP_STR];
> + u_int presskey_len = 0;
> + int i;
> + int abort = 0;
> +
> + if (sha_env_str == NULL)
> + sha_env_str = CONFIG_AUTOBOOT_STOP_STR_SHA256;
> +
> + /*
> + * Generate the binary value from the environment hash value
> + * so that we can compare this value with the computed hash
> + * from the user input
> + */
> + for (i = 0; i < SHA256_SUM_LEN; i++) {
> + char chr[3];
> +
> + strncpy(chr, &sha_env_str[i * 2], 2);
> + sha_env[i] = simple_strtoul(chr, NULL, 16);
> + }
> +

There is similar code in parse_verify_sum() in hash.c. Not sure if it
is worth putting it in a function, like:

hash_parse_string(const char *algo_name, cost char *str, u8 *result)

It could support other algorithms for autoboot in the future if that
became useful.

> + /*
> + * We don't know how long the stop-string is, so we need to
> + * generate the sha256 hash upon each input character and
> + * compare the value with the one saved in the environment
> + */
> + do {
> + if (tstc()) {
> + /* Check for input string overflow */
> + if (presskey_len >= MAX_DELAY_STOP_STR)
> + return 0;
> +
> + presskey[presskey_len++] = getc();
> +
> + /* Calculate sha256 upon each new char */
> + sha256_csum_wd((unsigned char *)presskey, presskey_len,
> + sha, CHUNKSZ_SHA256);

Could use hash_block()

> +
> + /* And check if sha matches saved value in env */
> + if (slow_equals(sha, sha_env, SHA256_SUM_LEN))
> + abort = 1;
> + }
> + } while (!abort && get_ticks() <= etime);
> +
> + return abort;
> +}
> +#else
> +static int passwd_abort(uint64_t etime)
> {
> int abort = 0;
> - uint64_t etime = endtick(bootdelay);
> struct {
> char *str;
> u_int len;
> @@ -42,7 +108,9 @@ static int abortboot_keyed(int bootdelay)
> }
> delaykey[] = {
> { .str = getenv("bootdelaykey"), .retry = 1 },
> + { .str = getenv("bootdelaykey2"), .retry = 1 },
> { .str = getenv("bootstopkey"), .retry = 0 },
> + { .str = getenv("bootstopkey2"), .retry = 0 },
> };
>
> char presskey[MAX_DELAY_STOP_STR];
> @@ -50,26 +118,21 @@ static int abortboot_keyed(int bootdelay)
> u_int presskey_max = 0;
> u_int i;
>
> -#ifndef CONFIG_ZERO_BOOTDELAY_CHECK
> - if (bootdelay == 0)
> - return 0;
> -#endif
> -
> -# ifdef CONFIG_AUTOBOOT_PROMPT
> - /*
> - * CONFIG_AUTOBOOT_PROMPT includes the %d for all boards.
> - * To print the bootdelay value upon bootup.
> - */
> - printf(CONFIG_AUTOBOOT_PROMPT, bootdelay);
> -# endif
> -
> # ifdef CONFIG_AUTOBOOT_DELAY_STR
> if (delaykey[0].str == NULL)
> delaykey[0].str = CONFIG_AUTOBOOT_DELAY_STR;
> # endif
> -# ifdef CONFIG_AUTOBOOT_STOP_STR
> +# ifdef CONFIG_AUTOBOOT_DELAY_STR2
> if (delaykey[1].str == NULL)
> - delaykey[1].str = CONFIG_AUTOBOOT_STOP_STR;
> + delaykey[1].str = CONFIG_AUTOBOOT_DELAY_STR2;
> +# endif
> +# ifdef CONFIG_AUTOBOOT_STOP_STR
> + if (delaykey[2].str == NULL)
> + delaykey[2].str = CONFIG_AUTOBOOT_STOP_STR;
> +# endif
> +# ifdef CONFIG_AUTOBOOT_STOP_STR2
> + if (delaykey[3].str == NULL)
> + delaykey[3].str = CONFIG_AUTOBOOT_STOP_STR2;
> # endif
>
> for (i = 0; i < sizeof(delaykey) / sizeof(delaykey[0]); i++) {
> @@ -119,6 +182,33 @@ static int abortboot_keyed(int bootdelay)
> }
> } while (!abort && get_ticks() <= etime);
>
> + return abort;
> +}
> +#endif
> +
> +/***************************************************************************
> + * Watch for 'delay' seconds for autoboot stop or autoboot delay string.
> + * returns: 0 - no key string, allow autoboot 1 - got key string, abort
> + */
> +static int abortboot_keyed(int bootdelay)
> +{
> + int abort;
> + uint64_t etime = endtick(bootdelay);
> +
> +#ifndef CONFIG_ZERO_BOOTDELAY_CHECK
> + if (bootdelay == 0)
> + return 0;
> +#endif
> +
> +# ifdef CONFIG_AUTOBOOT_PROMPT
> + /*
> + * CONFIG_AUTOBOOT_PROMPT includes the %d for all boards.
> + * To print the bootdelay value upon bootup.
> + */
> + printf(CONFIG_AUTOBOOT_PROMPT, bootdelay);
> +# endif
> +
> + abort = passwd_abort(etime);
> if (!abort)
> debug_bootkeys("key timeout\n");
>
> --
> 2.4.0
>

Regards,
Simon