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

Bartosz Golaszewski
2024-02-06T11:57:11Z

On Thu, 1 Feb 2024 at 17:11, Konrad Dybcio <konrad.dybcio@linaro.org> wrote:
>
> 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@quicinc.com>
> > Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
> > ---
> > 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?
>

I second this. Please look at other implementations.

Bart

> Konrad
>