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

On Fri, 10 Mar 2023 at 01:13, Jeffrey Kardatzke jkardatzke@chromium.org 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@chromium.org Signed-off-by: Jeffrey Kardatzke jkardatzke@google.com

---

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