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

[Jens Wiklander]

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<jean-michel.delorme@xxxxxx> and
> >   Emmanuel MICHEL <emmanuel.michel@xxxxxx>
> > * "Generic TrustZone Driver" by Javier GonzÃlez <javier@xxxxxxxxxxx>
> 
> 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