Re: [PATCH v4 02/15] qcom_scm: scm call for deriving a software secret

[Konrad Dybcio]

On 28.01.2024 00:14, Gaurav Kashyap wrote:
> Inline storage encryption may require deriving a software
> secret from storage keys added to the kernel.
> 
> For non-wrapped keys, this can be directly done in the kernel as
> keys are in the clear.
> 
> However, hardware wrapped keys can only be unwrapped by the wrapping
> entity. In case of Qualcomm's wrapped key solution, this is done by
> the Hardware Key Manager (HWKM) from Trustzone.
> Hence, adding a new SCM call which in the end provides a hook
> to the software secret crypto profile API provided by the block
> layer.
> 
> Signed-off-by: Gaurav Kashyap <quic_gaurkash@xxxxxxxxxxx>
> Tested-by: Neil Armstrong <neil.armstrong@xxxxxxxxxx>
> ---
>  drivers/firmware/qcom/qcom_scm.c       | 65 ++++++++++++++++++++++++++
>  drivers/firmware/qcom/qcom_scm.h       |  1 +
>  include/linux/firmware/qcom/qcom_scm.h |  2 +
>  3 files changed, 68 insertions(+)
> 
> diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
> index 7e17fd662bda..4882f8a36453 100644
> --- a/drivers/firmware/qcom/qcom_scm.c
> +++ b/drivers/firmware/qcom/qcom_scm.c
> @@ -1220,6 +1220,71 @@ int qcom_scm_ice_set_key(u32 index, const u8 *key, u32 key_size,
>  }
>  EXPORT_SYMBOL_GPL(qcom_scm_ice_set_key);
>  
> +/**
> + * qcom_scm_derive_sw_secret() - Derive software secret from wrapped key
> + * @wkey: the hardware wrapped key inaccessible to software
> + * @wkey_size: size of the wrapped key
> + * @sw_secret: the secret to be derived which is exactly the secret size
> + * @sw_secret_size: size of the sw_secret
> + *
> + * Derive a software secret from a hardware wrapped key for software crypto
> + * operations.
> + * For wrapped keys, the key needs to be unwrapped, in order to derive a
> + * software secret, which can be done in the hardware from a secure execution
> + * environment.
> + *
> + * For more information on sw secret, please refer to "Hardware-wrapped keys"
> + * section of Documentation/block/inline-encryption.rst.
> + *
> + * Return: 0 on success; -errno on failure.
> + */
> +int qcom_scm_derive_sw_secret(const u8 *wkey, size_t wkey_size,
> +			      u8 *sw_secret, size_t sw_secret_size)
> +{
> +	struct qcom_scm_desc desc = {
> +		.svc = QCOM_SCM_SVC_ES,
> +		.cmd =  QCOM_SCM_ES_DERIVE_SW_SECRET,
> +		.arginfo = QCOM_SCM_ARGS(4, QCOM_SCM_RW,
> +					 QCOM_SCM_VAL, QCOM_SCM_RW,
> +					 QCOM_SCM_VAL),
> +		.args[1] = wkey_size,
> +		.args[3] = sw_secret_size,
> +		.owner = ARM_SMCCC_OWNER_SIP,
> +	};
> +
> +	void *secret_buf;
> +	void *wkey_buf;
> +	int ret;
> +
> +	wkey_buf = qcom_tzmem_alloc(__scm->mempool, wkey_size, GFP_KERNEL);
> +	if (!wkey_buf)
> +		return -ENOMEM;
> +
> +	secret_buf = qcom_tzmem_alloc(__scm->mempool, sw_secret_size, GFP_KERNEL);
> +	if (!secret_buf) {
> +		ret = -ENOMEM;
> +		goto err_free_wrapped;
> +	}
> +
> +	memcpy(wkey_buf, wkey, wkey_size);
> +	desc.args[0] = qcom_tzmem_to_phys(wkey_buf);
> +	desc.args[2] = qcom_tzmem_to_phys(secret_buf);
> +
> +	ret = qcom_scm_call(__scm->dev, &desc, NULL);
> +	if (!ret)
> +		memcpy(sw_secret, secret_buf, sw_secret_size);
> +
> +	memzero_explicit(secret_buf, sw_secret_size);
> +	qcom_tzmem_free(secret_buf);
> +
> +err_free_wrapped:
> +	memzero_explicit(wkey_buf, wkey_size);
> +	qcom_tzmem_free(wkey_buf);
__free(qcom_tzmem) attribute instead?

Konrad