tee: optee: Add SMC for loading OP-TEE image

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 <[e-mail]>
Reviewed-by: Sumit Garg <[e-mail]>
Signed-off-by: Jeffrey Kardatzke <[e-mail]>
Signed-off-by: Jens Wiklander <[e-mail]>

Documentation/staging/tee.rst

···
 building block for OP-TEE OS in secure world to implement the top half and
 bottom half style of device drivers.
 
+OPTEE_INSECURE_LOAD_IMAGE Kconfig option
+----------------------------------------
+
+The OPTEE_INSECURE_LOAD_IMAGE Kconfig option enables the ability to load the
+BL32 OP-TEE image from the kernel after the kernel boots, rather than loading
+it from the firmware before the kernel boots. This also requires enabling the
+corresponding option in Trusted Firmware for Arm. The Trusted Firmware for Arm
+documentation [8] explains the security threat associated with enabling this as
+well as mitigations at the firmware and platform level.
+
+There are additional attack vectors/mitigations for the kernel that should be
+addressed when using this option.
+
+1. Boot chain security.
+
+   * Attack vector: Replace the OP-TEE OS image in the rootfs to gain control of
+     the system.
+
+   * Mitigation: There must be boot chain security that verifies the kernel and
+     rootfs, otherwise an attacker can modify the loaded OP-TEE binary by
+     modifying it in the rootfs.
+
+2. Alternate boot modes.
+
+   * Attack vector: Using an alternate boot mode (i.e. recovery mode), the
+     OP-TEE driver isn't loaded, leaving the SMC hole open.
+
+   * Mitigation: 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.
+
+3. Attacks prior to SMC invocation.
+
+   * Attack vector: Code that is executed prior to issuing the SMC call to load
+     OP-TEE can be exploited to then load an alternate OS image.
+
+   * Mitigation: 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 (e.g. USB).
+
+4. Blocking SMC call to load OP-TEE.
+
+   * Attack vector: Prevent the driver from being probed, so the SMC call to
+     load OP-TEE isn't executed when desired, leaving it open to being executed
+     later and loading a modified OS.
+
+   * Mitigation: It is recommended to build the OP-TEE driver as builtin driver
+     rather than as a module to prevent exploits that may cause the module to
+     not be loaded.
+
 AMD-TEE driver
 ==============
 
···
 [6] include/linux/psp-tee.h
 
 [7] drivers/tee/amdtee/amdtee_if.h
+
+[8] https://trustedfirmware-a.readthedocs.io/en/latest/threat_model/threat_model.html

drivers/tee/optee/Kconfig

···
 	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
+
+	  Additional documentation on kernel security risks are at
+	  Documentation/staging/tee.rst.

drivers/tee/optee/optee_msg.h

···
  * 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
 
 /*

drivers/tee/optee/optee_smc.h

···
 };
 
 /*
+ * 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
  *
  * When called with OPTEE_SMC_CALL_WITH_RPC_ARG or

drivers/tee/optee/smc_abi.c

···
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/arm-smccc.h>
+#include <linux/cpuhotplug.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>
···
 	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 {
···
 		optee_disable_shm_cache(optee);
 }
 
+#ifdef CONFIG_OPTEE_INSECURE_LOAD_IMAGE
+
+#define OPTEE_FW_IMAGE "optee/tee.bin"
+
+static optee_invoke_fn *cpuhp_invoke_fn;
+
+static int optee_cpuhp_probe(unsigned int cpu)
+{
+	/*
+	 * Invoking a call on a CPU will cause OP-TEE to perform the required
+	 * setup for that CPU. Just invoke the call to get the UID since that
+	 * has no side effects.
+	 */
+	if (optee_msg_api_uid_is_optee_api(cpuhp_invoke_fn))
+		return 0;
+	else
+		return -EINVAL;
+}
+
+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;
+	int hp_state;
+
+	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 = 0;
+	data_pa_low = 0;
+	data_size_high = 0;
+	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);
+	if (!rc)
+		rc = res.a0;
+	if (fw)
+		release_firmware(fw);
+	kfree(data_buf);
+
+	if (!rc) {
+		/*
+		 * We need to initialize OP-TEE on all other running cores as
+		 * well. Any cores that aren't running yet will get initialized
+		 * when they are brought up by the power management functions in
+		 * TF-A which are registered by the OP-TEE SPD. Due to that we
+		 * can un-register the callback right after registering it.
+		 */
+		cpuhp_invoke_fn = invoke_fn;
+		hp_state = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "optee:probe",
+					     optee_cpuhp_probe, NULL);
+		if (hp_state < 0) {
+			pr_warn("Failed with CPU hotplug setup for OP-TEE");
+			return -EINVAL;
+		}
+		cpuhp_remove_state(hp_state);
+		cpuhp_invoke_fn = NULL;
+	}
+
+	return rc;
+}
+#else
+static inline int optee_load_fw(struct platform_device *pdev,
+				optee_invoke_fn *invoke_fn)
+{
+	return 0;
+}
+#endif
+
 static int optee_probe(struct platform_device *pdev)
 {
 	optee_invoke_fn *invoke_fn;
···
 	invoke_fn = get_invoke_func(&pdev->dev);
 	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");