[PATCH v10] tee: optee: Add SMC for loading OP-TEE image
From: Jeffrey Kardatzke
Date: Wed Mar 15 2023 - 16:20:56 EST
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 v10:
- Fixed tee.rst documentation formatting
Changes in v9:
- Add CPU hotplug callback to init on all cores at startup
Changes in v8:
- Renamed params and fixed alignment issue
Changes in v7:
- Added documentation to Documentation/staging/tee.rst
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
Documentation/staging/tee.rst | 41 +++++++++++
drivers/tee/optee/Kconfig | 17 +++++
drivers/tee/optee/optee_msg.h | 12 +++
drivers/tee/optee/optee_smc.h | 24 ++++++
drivers/tee/optee/smc_abi.c | 134 ++++++++++++++++++++++++++++++++++
5 files changed, 228 insertions(+)
diff --git a/Documentation/staging/tee.rst b/Documentation/staging/tee.rst
index 498343c7ab08..6e90ee726880 100644
--- a/Documentation/staging/tee.rst
+++ b/Documentation/staging/tee.rst
@@ -214,6 +214,47 @@ call is done from the thread assisting the interrupt handler. This is a
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 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
+
+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.
+ Migitation: 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 an included
+ driver rather than a module to prevent exploits that may cause the module to
+ not be loaded.
+
AMD-TEE driver
==============
diff --git a/drivers/tee/optee/Kconfig b/drivers/tee/optee/Kconfig
index f121c224e682..70898bbd5809 100644
--- a/drivers/tee/optee/Kconfig
+++ b/drivers/tee/optee/Kconfig
@@ -7,3 +7,20 @@ 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
+
+ Additional documentation on kernel security risks are at
+ Documentation/staging/tee.rst.
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..fcbcd0c0c3aa 100644
--- a/drivers/tee/optee/smc_abi.c
+++ b/drivers/tee/optee/smc_abi.c
@@ -7,10 +7,13 @@
#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>
@@ -1149,6 +1152,22 @@ 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 +1373,117 @@ 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 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 = 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);
+ 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;
@@ -1372,6 +1502,10 @@ 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