Subject: [PATCH v13 2/5] tee: generic TEE subsystem

From: [email-masked] (Jens Wiklander)
Date: Thu, 19 Jan 2017 17:45:43 +0100

Hi Arnd,

This is the old v13 patch set you're commenting, but it doesn't matter
it's essentially identical to v14 in this patch.

On Wed, Jan 18, 2017 at 09:19:25PM +0100, Arnd Bergmann wrote:
> On Friday, November 18, 2016 3:51:37 PM CET Jens Wiklander wrote:
> > Initial patch for generic TEE subsystem.
> > This subsystem provides:
> > * Registration/un-registration of TEE drivers.
> > * Shared memory between normal world and secure world.
> > * Ioctl interface for interaction with user space.
> > * Sysfs implementation_id of TEE driver
> > 
> > A TEE (Trusted Execution Environment) driver is a driver that interfaces
> > with a trusted OS running in some secure environment, for example,
> > TrustZone on ARM cpus, or a separate secure co-processor etc.
> > 
> > The TEE subsystem can serve a TEE driver for a Global Platform compliant
> > TEE, but it's not limited to only Global Platform TEEs.
> > 
> > This patch builds on other similar implementations trying to solve
> > the same problem:
> > * "optee_linuxdriver" by among others
> >   Jean-michel DELORME<[email-masked]> and
> >   Emmanuel MICHEL <[email-masked]>
> > * "Generic TrustZone Driver" by Javier Gonz?lez <[email-masked]>
> 
> Can you give an example for a system that would contain more than one
> TEE? I see that you support dynamic registration, and it's clear that
> there can be more than one type of TEE, but why would one have more
> than one at a time, and why not more than 32?

I know that ST has systems where there's one TEE in TrustZone and
another TEE on a separate secure co-processor. If you have several TEEs
it's probably because they have different capabilities (performance
versus level of security). Just going beyond two or three different
levels of security with different TEEs sounds a bit extreme, so a
maximum of 32 or 16 should be fairly safe. If it turns out I'm wrong in
this assumption it's not that hard to correct it.

> 
> > +static int tee_ioctl_invoke(struct tee_context *ctx,
> > +			    struct tee_ioctl_buf_data __user *ubuf)
> > +{
> > +	int rc;
> > +	size_t n;
> > +	struct tee_ioctl_buf_data buf;
> > +	struct tee_ioctl_invoke_arg __user *uarg;
> > +	struct tee_ioctl_invoke_arg arg;
> > +	struct tee_ioctl_param __user *uparams = NULL;
> > +	struct tee_param *params = NULL;
> > +
> > +	if (!ctx->teedev->desc->ops->invoke_func)
> > +		return -EINVAL;
> > +
> > +	if (copy_from_user(&buf, ubuf, sizeof(buf)))
> > +		return -EFAULT;
> > +
> > +	if (buf.buf_len > TEE_MAX_ARG_SIZE ||
> > +	    buf.buf_len < sizeof(struct tee_ioctl_invoke_arg))
> > +		return -EINVAL;
> > +
> > +	uarg = (struct tee_ioctl_invoke_arg __user *)(unsigned long)buf.buf_ptr;
> 
> u64_to_user_ptr()

Thanks, that's convenient.

> 
> > +	if (copy_from_user(&arg, uarg, sizeof(arg)))
> > +		return -EFAULT;
> > +
> > +	if (sizeof(arg) + TEE_IOCTL_PARAM_SIZE(arg.num_params) != buf.buf_len)
> > +		return -EINVAL;
> > +
> > +	if (arg.num_params) {
> > +		params = kcalloc(arg.num_params, sizeof(struct tee_param),
> > +				 GFP_KERNEL);
> > +		if (!params)
> > +			return -ENOMEM;
> 
> It would be good to have an upper bound on the number of parameters
> to limit the size of the memory allocation here.

This is already limited due to:

The test with: buf.buf_len > TEE_MAX_ARG_SIZE

And then another test that the number of parameters matches the buffer size
with: sizeof(arg) + TEE_IOCTL_PARAM_SIZE(arg.num_params) != buf.buf_len

> 
> > +int tee_device_register(struct tee_device *teedev)
> > +{
> > +	int rc;
> > +
> > +	/*
> > +	 * If the teedev already is registered, don't do it again. It's
> > +	 * obviously an error to try to register twice, but if we return
> > +	 * an error we'll force the driver to remove the teedev.
> > +	 */
> > +	if (teedev->flags & TEE_DEVICE_FLAG_REGISTERED) {
> > +		dev_err(&teedev->dev, "attempt to register twice\n");
> > +		return 0;
> > +	}
> 
> I don't understand what you are protecting against here.
> How would we get to this function twice for the same device?
> 
> Could you change the caller so it doesn't happen?

Yes the caller can be changed, I'll return an error instead (and remove
the comment).

> 
> > +/**
> > + * struct tee_ioctl_param - parameter
> > + * @attr: attributes
> > + * @memref: a memory reference
> > + * @value: a value
> > + *
> > + * @attr & TEE_PARAM_ATTR_TYPE_MASK indicates if memref or value is used in
> > + * the union. TEE_PARAM_ATTR_TYPE_VALUE_* indicates value and
> > + * TEE_PARAM_ATTR_TYPE_MEMREF_* indicates memref. TEE_PARAM_ATTR_TYPE_NONE
> > + * indicates that none of the members are used.
> > + */
> > +struct tee_ioctl_param {
> > +	__u64 attr;
> > +	union {
> > +		struct tee_ioctl_param_memref memref;
> > +		struct tee_ioctl_param_value value;
> > +	} u;
> > +};
> > +
> > +#define TEE_IOCTL_UUID_LEN		16
> > +
> 
> Having a union in an ioctl argument seems odd. Have you considered
> using two different ioctl command numbers depending on the type?

struct tee_ioctl_param is used as an array and some parameters can be
memrefs while other are values.

> 
> > +/**
> > + * struct tee_iocl_supp_send_arg - Send a response to a received request
> > + * @ret:	[out] return value
> > + * @num_params	[in] number of parameters following this struct
> > + */
> > +struct tee_iocl_supp_send_arg {
> > +	__u32 ret;
> > +	__u32 num_params;
> > +	/*
> > +	 * this struct is 8 byte aligned since the 'struct tee_ioctl_param'
> > +	 * which follows requires 8 byte alignment.
> > +	 *
> > +	 * Commented out element used to visualize the layout dynamic part
> > +	 * of the struct. This field is not available at all if
> > +	 * num_params == 0.
> > +	 *
> > +	 * struct tee_ioctl_param params[num_params];
> > +	 */
> > +} __aligned(8);
> 
> I'd make that 
> 
> 	struct tee_ioctl_param params[0];
> 
> as wel here, as I also commented in patch 3 that has a similar structure.

I'm concerned that this may cause warnings when compiling for user space
depending on compiler and options. Am I too cautious here?
 
Thanks,
Jens