Message-ID: <CA+ddPcMvY-VYVy6O0XUdKBEVw6ncqfgzw9C1BCRkqg0_G3DSfQ@mail.gmail.com>
Date:   Tue, 7 Mar 2023 11:39:17 -0800
From:   Jeffrey Kardatzke <jkardatzke@...omium.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     op-tee@...ts.trustedfirmware.org,
        Jens Wiklander <jens.wiklander@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] tee: optee: Add SMC for loading OP-TEE image

On Tue, Mar 7, 2023 at 6:04 AM Sumit Garg <sumit.garg@...aro.org> wrote:
>
> On Tue, 7 Mar 2023 at 00:26, Jeffrey Kardatzke <jkardatzke@...omium.org> wrote:
> >
> >
> >
> > On Mon, Mar 6, 2023 at 4:34 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> >>
> >> On Fri, 3 Mar 2023 at 01:37, Jeffrey Kardatzke <jkardatzke@...omium.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 ARM that supports this.
> >>
> >> s/Trusted Firmware for ARM/Trusted Firmware for Armv8 and above architectures/
> >>
> > Fixed in next patch.
> >>
> >> This commit description tells me what you have implemented but not
> >> why? The motivation for this change should be included here along with
> >> security implications.
> >>
> > Fixed in next patch. I added another sentence on this and then linked to the TF-A documentation that explains the threat model and security implications. Enabling the kernel feature itself won't create any security concerns; it's only if TF-A has the feature enabled is there a risk introduced.
>
> Do you have a scenario where this feature would be at all useful in
> the kernel without it being enabled in TF-A?

No
>
>
> So once you enable this feature in TF-A, isn't the kernel included in
> the trust boundary for the secure world to do the right thing? IOW,
> mitigations for potential attack vectors have to be implemented in the
> kernel such as (from top of my head):
>
> - The OP-TEE kernel module has to be signature checked.
> - The OP-TEE firmware has to be loaded from a RO filesystem which has
> been signature checked.
> - How can we prevent other system entities like DMA capable devices
> from mangling RAM where OP-TEE firmware is loaded during SMC call?
> - How can we ensure that the OP-TEE firmware is loaded at the earliest
> point before exposing it to the user-space attack vectors?

This is covered in the TF-A documentation linked to by this statement
in the mitigations section: "When enabling the option
OPTEE_ALLOW_SMC_LOAD, the non-secure OS must be considered a closed
platform up until the point the SMC can be invoked to load OP-TEE."
It's also expanded on in the linked section here:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/components/spd/optee-dispatcher.rst

For ChromeOS, the entire kernel/rootfs is signature checked. Other DMA
devices could modify the memory where OP-TEE is loaded from...so the
platform needs to ensure all firmware/drivers are signed (which we
achieve by signing the entire rootfs). As for loading it early, it can
be included rather than being a module to achieve that (or it can be a
module and should be probed as early as possible after the kernel
loads).  This is definitely not intended to be "here's a checklist, if
you cover it then you're safe to use this option." Anybody using this
needs to evaluate their overall platform security and be sure this
fits with their model.
>
>
> >>
> >> >
> >> > Signed-off-by: Jeffrey Kardatzke <jkardatzke@...omium.org>
> >> > Signed-off-by: Jeffrey Kardatzke <jkardatzke@...gle.com>
> >> > ---
> >> >
> >> > 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     | 10 ++++
> >> >  drivers/tee/optee/optee_msg.h | 12 +++++
> >> >  drivers/tee/optee/optee_smc.h | 24 +++++++++
> >> >  drivers/tee/optee/smc_abi.c   | 96 +++++++++++++++++++++++++++++++++++
> >> >  4 files changed, 142 insertions(+)
> >> >
> >> > diff --git a/drivers/tee/optee/Kconfig b/drivers/tee/optee/Kconfig
> >> > index f121c224e682..f0f12b146add 100644
> >> > --- a/drivers/tee/optee/Kconfig
> >> > +++ b/drivers/tee/optee/Kconfig
> >> > @@ -7,3 +7,13 @@ config OPTEE
> >> >         help
> >> >           This implements the OP-TEE Trusted Execution Environment (TEE)
> >> >           driver.
> >> > +
> >> > +config OPTEE_LOAD_IMAGE
> >>
> >> Given the warning text attached to this config option, this should be
> >> renamed as OPTEE_INSECURE_LOAD_IMAGE. Also, the help text should be
> >> updated to reflect the security implication of this.
> >
> > OK, I renamed this to OPTEE_NONSECURE_LOAD_IMAGE (to reflect that it's loading the secure image from the non-secure world).
>
> No this has to be an insecure OP-TEE module configuration which
> currently only works for your platform threat model. I would also be
> in favour of a runtime warning if this option is enabled.

OK, renamed it to OPTEE_INSECURE_LOAD_IMAGE and added a runtime warning.
>
>
> > I also added more comments on the security implications and that full documentation is in TF-A.
> >>
> >>
> >> I would also like to see documentation updates regarding how your
> >> platform is mitigating potential threats in order to enable this
> >> option. It would enable others to make a concious decision if their
> >> threat model allows this option.
> >
> > This is already covered in the TF-A documentation. Let me know if you want me to link to that from here or not.
>
> I am looking for documentation regarding risk mitigations in the
> kernel (see above), basically how one should configure their kernel.
> Maybe I have overlooked it but I can't find that in the TF-A
> documentation.

It's not so much the kernel config...but the platform config overall.
This section linked to from the TF-A thread model describes that even
further:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/components/spd/optee-dispatcher.rst
>
>
> -Sumit
>
> >>
> >>
> >> > +       bool "Load OP-TEE image as firmware"
> >> > +       default n
> >> > +       depends on OPTEE
> >>
> >> IIRC, from earlier review threads you mentioned that this option is
> >> enabled when OP-TEE driver is built-in rather than loadable. Shouldn't
> >> we make that dependency explicit?
> >
> > That was an issue with a prior patch set, but has since been fixed. This can work as a built in driver or loadable module now.
> >>
> >>
> >> Also, this option only makes sense for ARM64 as in case of ARM the
> >> secure monitor is bundled into OP-TEE image and hence can't be loaded
> >> dynamically. We should make that dependency explicit too.
> >
> > Fixed.
> >>
> >>
> >> > +       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.
> >> > diff --git a/drivers/tee/optee/optee_msg.h b/drivers/tee/optee/optee_msg.h
> >> > index 70e9cc2ee96b..237d6fa9a6e8 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.
> >> > + * Represetned in 4 32-bit words in OPTEE_MSG_IMAGE_LOAD_UID_0,
> >>
> >> s/Represetned/Represented/
> >>
> > Fixed.
> >>
> >> > + * 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..14612edef8f2 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_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 +1372,80 @@ static void optee_shutdown(struct platform_device *pdev)
> >> >                 optee_disable_shm_cache(optee);
> >> >  }
> >> >
> >> > +#ifdef CONFIG_OPTEE_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.
> >> > +        */
> >> > +       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);
> >> > +
> >> > +       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 +1464,10 @@ 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.rc0.216.gc4246ad0f0-goog
> >> >

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