Re: [PATCH v6] tee: optee: Add SMC for loading OP-TEE image

[Sumit Garg]

On Fri, 10 Mar 2023 at 01:13, Jeffrey Kardatzke <jkardatzke@xxxxxxxxxxxx> wrote:
>
> Adds an SMC call that will pass an OP-TEE binary image to EL3 and
> instruct it to load it as the BL32 payload. This works in conjunction
> with a feature added to Trusted Firmware for ARMv8 and above
> architectures that supports this.
>
> The main purpose of this change is to facilitate updating the OP-TEE
> component on devices via a rootfs change rather than having to do a
> firmware update. Further details are linked to in the Kconfig file.
>
> Signed-off-by: Jeffrey Kardatzke <jkardatzke@xxxxxxxxxxxx>
> Signed-off-by: Jeffrey Kardatzke <jkardatzke@xxxxxxxxxx>
> ---
>
> Changes in v6:
> - Expanded Kconfig documentation
>
> Changes in v5:
> - Renamed config option
> - Added runtime warning when config is used
>
> Changes in v4:
> - Update commit message
> - Added more documentation
> - Renamed config option, added ARM64 dependency
>
> Changes in v3:
> - Removed state tracking for driver reload
> - Check UID of service to verify it needs image load
>
> Changes in v2:
> - Fixed compile issue when feature is disabled
> - Addressed minor comments
> - Added state tracking for driver reload
>
>  drivers/tee/optee/Kconfig     | 29 +++++++++++
>  drivers/tee/optee/optee_msg.h | 12 +++++
>  drivers/tee/optee/optee_smc.h | 24 +++++++++
>  drivers/tee/optee/smc_abi.c   | 97 +++++++++++++++++++++++++++++++++++
>  4 files changed, 162 insertions(+)
>
> diff --git a/drivers/tee/optee/Kconfig b/drivers/tee/optee/Kconfig
> index f121c224e682..8d4836c58486 100644
> --- a/drivers/tee/optee/Kconfig
> +++ b/drivers/tee/optee/Kconfig
> @@ -7,3 +7,32 @@ config OPTEE
>         help
>           This implements the OP-TEE Trusted Execution Environment (TEE)
>           driver.
> +
> +config OPTEE_INSECURE_LOAD_IMAGE
> +       bool "Load OP-TEE image as firmware"
> +       default n
> +       depends on OPTEE && ARM64
> +       help
> +         This loads the BL32 image for OP-TEE as firmware when the driver is
> +         probed. This returns -EPROBE_DEFER until the firmware is loadable from
> +         the filesystem which is determined by checking the system_state until
> +         it is in SYSTEM_RUNNING. This also requires enabling the corresponding
> +         option in Trusted Firmware for Arm. The documentation there explains
> +         the security threat associated with enabling this as well as
> +         mitigations at the firmware and platform level.
> +         https://trustedfirmware-a.readthedocs.io/en/latest/threat_model/threat_model.html
> +
> +         When utilizing this option, the following mitigations should be
> +         implemented to prevent attacks at the kernel level.
> +         1. There must be boot chain security that verifies the kernel and
> +            rootfs, otherwise an attacker can modify the loaded OP-TEE binary.
> +         2. It is recommended to build it as an included driver rather than
> +            a module to prevent exploits that may cause the module to not be
> +            loaded.
> +         3. If there are alternate methods of booting the device, such as a
> +            recovery mode, it should be ensured that the same mitigations are
> +            applied in that mode.
> +         4. The OP-TEE driver must be loaded before any potential attack
> +            vectors are opened up. This should include mounting of any
> +            modifiable filesystems, opening of network ports or communicating
> +            with external devices (such a USB).

This detailed threat model documentation belongs here [1] and it
should rather be in following format for every bullet point:

Attack vector: <>
Mitigation: <>

[1] https://docs.kernel.org/staging/tee.html?highlight=tee#op-tee-driver

> diff --git a/drivers/tee/optee/optee_msg.h b/drivers/tee/optee/optee_msg.h
> index 70e9cc2ee96b..e8840a82b983 100644
> --- a/drivers/tee/optee/optee_msg.h
> +++ b/drivers/tee/optee/optee_msg.h
> @@ -241,11 +241,23 @@ struct optee_msg_arg {
>   * 384fb3e0-e7f8-11e3-af63-0002a5d5c51b.
>   * Represented in 4 32-bit words in OPTEE_MSG_UID_0, OPTEE_MSG_UID_1,
>   * OPTEE_MSG_UID_2, OPTEE_MSG_UID_3.
> + *
> + * In the case where the OP-TEE image is loaded by the kernel, this will
> + * initially return an alternate UID to reflect that we are communicating with
> + * the TF-A image loading service at that time instead of OP-TEE. That UID is:
> + * a3fbeab1-1246-315d-c7c4-06b9c03cbea4.
> + * Represented in 4 32-bit words in OPTEE_MSG_IMAGE_LOAD_UID_0,
> + * OPTEE_MSG_IMAGE_LOAD_UID_1, OPTEE_MSG_IMAGE_LOAD_UID_2,
> + * OPTEE_MSG_IMAGE_LOAD_UID_3.
>   */
>  #define OPTEE_MSG_UID_0                        0x384fb3e0
>  #define OPTEE_MSG_UID_1                        0xe7f811e3
>  #define OPTEE_MSG_UID_2                        0xaf630002
>  #define OPTEE_MSG_UID_3                        0xa5d5c51b
> +#define OPTEE_MSG_IMAGE_LOAD_UID_0     0xa3fbeab1
> +#define OPTEE_MSG_IMAGE_LOAD_UID_1     0x1246315d
> +#define OPTEE_MSG_IMAGE_LOAD_UID_2     0xc7c406b9
> +#define OPTEE_MSG_IMAGE_LOAD_UID_3     0xc03cbea4
>  #define OPTEE_MSG_FUNCID_CALLS_UID     0xFF01
>
>  /*
> diff --git a/drivers/tee/optee/optee_smc.h b/drivers/tee/optee/optee_smc.h
> index 73b5e7760d10..7d9fa426505b 100644
> --- a/drivers/tee/optee/optee_smc.h
> +++ b/drivers/tee/optee/optee_smc.h
> @@ -104,6 +104,30 @@ struct optee_smc_call_get_os_revision_result {
>         unsigned long reserved1;
>  };
>
> +/*
> + * Load Trusted OS from optee/tee.bin in the Linux firmware.
> + *
> + * WARNING: Use this cautiously as it could lead to insecure loading of the
> + * Trusted OS.
> + * This SMC instructs EL3 to load a binary and execute it as the Trusted OS.
> + *
> + * Call register usage:
> + * a0 SMC Function ID, OPTEE_SMC_CALL_LOAD_IMAGE
> + * a1 Upper 32bit of a 64bit size for the payload
> + * a2 Lower 32bit of a 64bit size for the payload
> + * a3 Upper 32bit of the physical address for the payload
> + * a4 Lower 32bit of the physical address for the payload
> + *
> + * The payload is in the OP-TEE image format.
> + *
> + * Returns result in a0, 0 on success and an error code otherwise.
> + */
> +#define OPTEE_SMC_FUNCID_LOAD_IMAGE 2
> +#define OPTEE_SMC_CALL_LOAD_IMAGE \
> +       ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL, ARM_SMCCC_SMC_32, \
> +                          ARM_SMCCC_OWNER_TRUSTED_OS_END, \
> +                          OPTEE_SMC_FUNCID_LOAD_IMAGE)
> +
>  /*
>   * Call with struct optee_msg_arg as argument
>   *
> diff --git a/drivers/tee/optee/smc_abi.c b/drivers/tee/optee/smc_abi.c
> index a1c1fa1a9c28..00b6b69b6f79 100644
> --- a/drivers/tee/optee/smc_abi.c
> +++ b/drivers/tee/optee/smc_abi.c
> @@ -8,9 +8,11 @@
>
>  #include <linux/arm-smccc.h>
>  #include <linux/errno.h>
> +#include <linux/firmware.h>
>  #include <linux/interrupt.h>
>  #include <linux/io.h>
>  #include <linux/irqdomain.h>
> +#include <linux/kernel.h>
>  #include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/of.h>
> @@ -1149,6 +1151,22 @@ static bool optee_msg_api_uid_is_optee_api(optee_invoke_fn *invoke_fn)
>         return false;
>  }
>
> +#ifdef CONFIG_OPTEE_INSECURE_LOAD_IMAGE
> +static bool optee_msg_api_uid_is_optee_image_load(optee_invoke_fn *invoke_fn)
> +{
> +       struct arm_smccc_res res;
> +
> +       invoke_fn(OPTEE_SMC_CALLS_UID, 0, 0, 0, 0, 0, 0, 0, &res);
> +
> +       if (res.a0 == OPTEE_MSG_IMAGE_LOAD_UID_0 &&
> +          res.a1 == OPTEE_MSG_IMAGE_LOAD_UID_1 &&
> +          res.a2 == OPTEE_MSG_IMAGE_LOAD_UID_2 &&
> +          res.a3 == OPTEE_MSG_IMAGE_LOAD_UID_3)
> +               return true;
> +       return false;
> +}
> +#endif
> +
>  static void optee_msg_get_os_revision(optee_invoke_fn *invoke_fn)
>  {
>         union {
> @@ -1354,6 +1372,81 @@ static void optee_shutdown(struct platform_device *pdev)
>                 optee_disable_shm_cache(optee);
>  }
>
> +#ifdef CONFIG_OPTEE_INSECURE_LOAD_IMAGE
> +
> +#define OPTEE_FW_IMAGE "optee/tee.bin"
> +
> +static int optee_load_fw(struct platform_device *pdev,
> +                        optee_invoke_fn *invoke_fn)
> +{
> +       const struct firmware *fw = NULL;
> +       struct arm_smccc_res res;
> +       phys_addr_t data_pa;
> +       u8 *data_buf = NULL;
> +       u64 data_size;
> +       u32 data_pa_high, data_pa_low;
> +       u32 data_size_high, data_size_low;
> +       int rc;
> +
> +       if (!optee_msg_api_uid_is_optee_image_load(invoke_fn))
> +               return 0;
> +
> +       rc = request_firmware(&fw, OPTEE_FW_IMAGE, &pdev->dev);
> +       if (rc) {
> +               /*
> +                * The firmware in the rootfs will not be accessible until we
> +                * are in the SYSTEM_RUNNING state, so return EPROBE_DEFER until
> +                * that point.
> +                */
> +               if (system_state < SYSTEM_RUNNING)
> +                       return -EPROBE_DEFER;
> +               goto fw_err;
> +       }
> +
> +       data_size = fw->size;
> +       /*
> +        * This uses the GFP_DMA flag to ensure we are allocated memory in the
> +        * 32-bit space since TF-A cannot map memory beyond the 32-bit boundary.
> +        */
> +       data_buf = kmalloc(fw->size, GFP_KERNEL | GFP_DMA);
> +       if (!data_buf) {
> +               rc = -ENOMEM;
> +               goto fw_err;
> +       }
> +       memcpy(data_buf, fw->data, fw->size);
> +       data_pa = virt_to_phys(data_buf);
> +       reg_pair_from_64(&data_pa_high, &data_pa_low, data_pa);
> +       reg_pair_from_64(&data_size_high, &data_size_low, data_size);
> +       goto fw_load;
> +
> +fw_err:
> +       pr_warn("image loading failed\n");
> +       data_pa_high = data_pa_low = data_size_high = data_size_low = 0;
> +
> +fw_load:
> +       /*
> +        * Always invoke the SMC, even if loading the image fails, to indicate
> +        * to EL3 that we have passed the point where it should allow invoking
> +        * this SMC.
> +        */
> +       pr_warn("OP-TEE image loaded from kernel, this can be insecure");
> +       invoke_fn(OPTEE_SMC_CALL_LOAD_IMAGE, data_size_high, data_size_low,
> +                 data_pa_high, data_pa_low, 0, 0, 0, &res);

Apart from the security considerations discussed, I see an issue with
the implementation here. Here you only initialize OP-TEE on *CPUX*
that is performing OP-TEE probe. IIRC, it is required for that CPUX to
be primary CPU0. How do we ensure that here?

Also, I observe from the TF-A patch that you are doing lazy OP-TEE
initialization for other secondary CPUs. IOW, if there is an OP-TEE
SMC call invoked for a particular CPU then only you invoke
opteed_cpu_on_finish_handler(0) once. This can be a bit unsafe as
OP-TEE hasn't setup its context for those CPU which may involve some
CPU specific security bits too such as:
- GIC CPU interface initialization, secure vs non-secure interrupts.
- Any platform and CPU specific TrustZone configuration.

I would have rather expected you to utilize cpuhp_setup_state() and
friends to initialize OP-TEE for secondary CPUs during boot instead
which is safe as per your platform threat model.

-Sumit

> +       if (!rc)
> +               rc = res.a0;
> +       if (fw)
> +               release_firmware(fw);
> +       kfree(data_buf);
> +
> +       return rc;
> +}
> +#else
> +static inline int optee_load_fw(struct platform_device *__unused1,
> +               optee_invoke_fn *__unused2) {
> +       return 0;
> +}
> +#endif
> +
>  static int optee_probe(struct platform_device *pdev)
>  {
>         optee_invoke_fn *invoke_fn;
> @@ -1372,6 +1465,10 @@ static int optee_probe(struct platform_device *pdev)
>         if (IS_ERR(invoke_fn))
>                 return PTR_ERR(invoke_fn);
>
> +       rc = optee_load_fw(pdev, invoke_fn);
> +       if (rc)
> +               return rc;
> +
>         if (!optee_msg_api_uid_is_optee_api(invoke_fn)) {
>                 pr_warn("api uid mismatch\n");
>                 return -EINVAL;
> --
> 2.40.0.rc1.284.g88254d51c5-goog
>