Re: [PATCH BlueZ v2 1/3] src: add HMAC-SHA256 to bt_crypto using kernel AF_ALG

Hi Pakrohk,

On Tue, Jul 21, 2026 at 10:17 AM Pakrohk <[email protected]> wrote:
>
> From: Pakrohk <[email protected]>
>
> Add HMAC-SHA256 computation to the bt_crypto library using the Linux
> kernel AF_ALG interface. This follows the exact same pattern as the
> existing cmac(aes) implementation.
>
> The HMAC-SHA256 function is needed for verifying external quirk profile
> signatures. The previous implementation used popen("openssl dgst ...")
> which had command injection risks and an openssl CLI dependency.
>
> Using kernel AF_ALG:
>   - No external library dependencies
>   - Matches existing BlueZ crypto patterns
>   - No shell invocation or temp files
>   - Works in bluetoothd's process context

Well I guess kernel crypto doesn't want us to add new usage like this
since AF_ALG is to be considered deprecated, even though I have to
agree it is much simpler and probably safer than using an external
crypto library.

> The hmac(sha256) algorithm is available in all modern Linux kernels
> via the crypto API (/proc/crypto).
>
> Signed-off-by: Pakrohk <[email protected]>
> diff --git a/src/shared/crypto.c b/src/shared/crypto.c
> index cb99116..f5883bd 100644
> --- a/src/shared/crypto.c
> +++ b/src/shared/crypto.c
> @@ -69,6 +69,7 @@ struct bt_crypto {
>         int ecb_aes;
>         int urandom;
>         int cmac_aes;
> +       int hmac_sha256;
>  };
>
>  static int urandom_setup(void)
> @@ -126,6 +127,28 @@ static int cmac_aes_setup(void)
>         return fd;
>  }
>
> +static int hmac_sha256_setup(void)
> +{
> +       struct sockaddr_alg salg;
> +       int fd;
> +
> +       fd = socket(PF_ALG, SOCK_SEQPACKET | SOCK_CLOEXEC, 0);
> +       if (fd < 0)
> +               return -1;
> +
> +       memset(&salg, 0, sizeof(salg));
> +       salg.salg_family = AF_ALG;
> +       strcpy((char *) salg.salg_type, "hash");
> +       strcpy((char *) salg.salg_name, "hmac(sha256)");
> +
> +       if (bind(fd, (struct sockaddr *) &salg, sizeof(salg)) < 0) {
> +               close(fd);
> +               return -1;
> +       }
> +
> +       return fd;
> +}
> +
>  static struct bt_crypto *singleton;
>
>  struct bt_crypto *bt_crypto_new(void)
> @@ -159,6 +182,16 @@ struct bt_crypto *bt_crypto_new(void)
>                 return NULL;
>         }
>
> +       singleton->hmac_sha256 = hmac_sha256_setup();
> +       if (singleton->hmac_sha256 < 0) {
> +               close(singleton->cmac_aes);
> +               close(singleton->urandom);
> +               close(singleton->ecb_aes);
> +               free(singleton);
> +               singleton = NULL;
> +               return NULL;
> +       }
> +
>         return bt_crypto_ref(singleton);
>  }
>
> @@ -183,6 +216,7 @@ void bt_crypto_unref(struct bt_crypto *crypto)
>         close(crypto->urandom);
>         close(crypto->ecb_aes);
>         close(crypto->cmac_aes);
> +       close(crypto->hmac_sha256);
>
>         free(crypto);
>         singleton = NULL;
> @@ -1005,3 +1039,35 @@ bool bt_crypto_sirk(struct bt_crypto *crypto, const char *str, uint16_t vendor,
>         /* Encrypt sirk using k as LTK with sef function */
>         return bt_crypto_sef(crypto, k, sirk_plaintext, sirk);
>  }
> +
> +bool bt_crypto_hmac_sha256(struct bt_crypto *crypto,
> +                               const uint8_t *key, size_t key_len,
> +                               const uint8_t *msg, size_t msg_len,
> +                               uint8_t res[32])
> +{
> +       ssize_t len;
> +       int fd;
> +
> +       if (!crypto)
> +               return false;
> +
> +       fd = alg_new(crypto->hmac_sha256, key, key_len);
> +       if (fd < 0)
> +               return false;
> +
> +       len = send(fd, msg, msg_len, 0);
> +       if (len < 0) {
> +               close(fd);
> +               return false;
> +       }
> +
> +       len = read(fd, res, 32);
> +       if (len < 0) {
> +               close(fd);
> +               return false;
> +       }
> +
> +       close(fd);
> +
> +       return true;
> +}
> diff --git a/src/shared/crypto.h b/src/shared/crypto.h
> index d85f807..6b234bc 100644
> --- a/src/shared/crypto.h
> +++ b/src/shared/crypto.h
> @@ -62,3 +62,8 @@ bool bt_crypto_sirk(struct bt_crypto *crypto, const char *str, uint16_t vendor,
>                         uint8_t sirk[16]);
>  bool bt_crypto_rsi(struct bt_crypto *crypto, const uint8_t sirk[16],
>                                         uint8_t rsi[6]);
> +
> +bool bt_crypto_hmac_sha256(struct bt_crypto *crypto,
> +                               const uint8_t *key, size_t key_len,
> +                               const uint8_t *msg, size_t msg_len,
> +                               uint8_t res[32]);
>


-- 
Luiz Augusto von Dentz