Re: [PATCH 3/3][RFC] tools: create power/crypto utility

Eric Biggers
2018-06-20T17:41:49Z

Hi Chen,

On Wed, Jun 20, 2018 at 05:40:51PM +0800, Chen Yu wrote:
> crypto_hibernate is a user-space utility to generate
> 512bits AES key and pass it to the kernel via ioctl
> for hibernation encryption.(We can also add the key
> into kernel via keyctl if necessary, but currently
> using ioctl seems to be more straightforward as we
> need both the key and salt transit).
>
> The key derivation is based on a simplified implementation
> of PBKDF2[1] in e2fsprogs - both the key length and the hash
> bytes are the same - 512bits. crypto_hibernate will firstly
> probe the user for passphrase and get salt from kernel, then
> uses them to generate a 512bits AES key based on PBKDF2.

What is a "512bits AES key"? Do you mean AES-256-XTS (which takes a 512-bit
key, which the XTS mode internally splits into two keys)? Do you allow for
other algorithms, or is it hardcoded to AES-256-XTS? What if someone wants to
use a different algorithm?

BTW, it's difficult to review this with only patch 3/3 Cc'ed to me, as there is
no context about the problem you are trying to solve and what your actual
proposed kernel changes are. I suggest Cc'ing linux-crypto on all 3 patches.

A few more comments below, from a very brief reading of the code:

[...]
> +
> +#define PBKDF2_ITERATIONS 0xFFFF
> +#define SHA512_BLOCKSIZE 128
> +#define SHA512_LENGTH 64
> +#define SALT_BYTES 16
> +#define SYM_KEY_BYTES SHA512_LENGTH
> +#define TOTAL_USER_INFO_LEN (SALT_BYTES+SYM_KEY_BYTES)
> +#define MAX_PASSPHRASE_SIZE 1024
> +
> +struct hibernation_crypto_keys {
> + char derived_key[SYM_KEY_BYTES];
> + char salt[SALT_BYTES];
> + bool valid;
> +};
> +
> +struct hibernation_crypto_keys hib_keys;
> +
> +static char *get_key_ptr(void)
> +{
> + return hib_keys.derived_key;
> +}
> +
> +static char *get_salt_ptr(void)
> +{
> + return hib_keys.salt;
> +}
[...]
> +
> +
> +#define HIBERNATE_SALT_READ _IOW('C', 3, struct hibernation_crypto_keys)
> +#define HIBERNATE_KEY_WRITE _IOW('C', 4, struct hibernation_crypto_keys)

Why are the ioctl numbers defined based on the size of 'struct
hibernation_crypto_keys'? It's not a UAPI structure, right?

> +
> +static void get_passphrase(char *passphrase, int len)
> +{
> + char *p;
> + struct termios current_settings;
> +
> + assert(len > 0);
> + disable_echo(&current_settings);
> + p = fgets(passphrase, len, stdin);
> + tcsetattr(0, TCSANOW, &current_settings);
> + printf("\n");
> + if (!p) {
> + printf("Aborting.\n");
> + exit(1);
> + }
> + p = strrchr(passphrase, '\n');
> + if (!p)
> + p = passphrase + len - 1;
> + *p = '\0';
> +}
> +
> +#define CRYPTO_FILE "/dev/crypto_hibernate"
> +
> +static int write_keys(void)
> +{
> + int fd;
> +
> + fd = open(CRYPTO_FILE, O_RDWR);
> + if (fd < 0) {
> + printf("Cannot open device file...\n");
> + return -EINVAL;
> + }
> + ioctl(fd, HIBERNATE_KEY_WRITE, get_key_ptr());
> + return 0;

No error checking on the ioctl?

Also, how is the kernel supposed to know how long the key is, and which
algorithm it's supposed to be used for? I assume it's hardcoded to AES-256-XTS?
What if someone wants to use a different algorithm?

> +}
> +
> +static int read_salt(void)
> +{
> + int fd;
> +
> + fd = open(CRYPTO_FILE, O_RDWR);
> + if (fd < 0) {
> + printf("Cannot open device file...\n");
> + return -EINVAL;
> + }
> + ioctl(fd, HIBERNATE_SALT_READ, get_salt_ptr());
> + return 0;
> +}

No error checking on the ioctl?

> +int main(int argc, char *argv[])
> +{
> + int opt, option_index = 0;
> + char in_passphrase[MAX_PASSPHRASE_SIZE];
> +
> + while ((opt = getopt_long_only(argc, argv, "+p:s:h",
> + NULL, &option_index)) != -1) {
> + switch (opt) {
> + case 'p':
> + {
> + char *p = optarg;
> +
> + if (strlen(p) >= (MAX_PASSPHRASE_SIZE - 1)) {
> + printf("Please provide passphrase less than %d bytes.\n",
> + MAX_PASSPHRASE_SIZE);
> + exit(1);
> + }
> + strcpy(in_passphrase, p);

I haven't read this super closely, but this really looks like an off-by-one
error. It seems you intended MAX_PASSPHRASE_SIZE to include a null terminator,
so the correct check would be 'strlen(p) >= MAX_PASSPHRASE_SIZE'.

> + }
> + break;
> + case 's':
> + {
> + char *p = optarg;
> +
> + if (strlen(p) != (SALT_BYTES - 1)) {
> + printf("Please provide salt with len less than %d bytes.\n",
> + SALT_BYTES);
> + exit(1);
> + }
> + strcpy(get_salt_ptr(), p);
> + }
> + break;

Salts don't need to be human-readable. How about making the salt binary? So, a
salt specified on the command-line would be hex.

Eric