Subject: [tpmdd-devel] [RFC PATCH 1/2] tee: generic TEE subsystem

From: [email-masked] (Jason Gunthorpe)
Date: Sat, 18 Apr 2015 11:29:23 -0600

On Sat, Apr 18, 2015 at 10:01:47AM +0100, Russell King - ARM Linux wrote:
> On Fri, Apr 17, 2015 at 10:30:54AM -0600, Jason Gunthorpe wrote:
> > On Fri, Apr 17, 2015 at 09:50:56AM +0200, Jens Wiklander wrote:
> > > +	teedev = devm_kzalloc(dev, sizeof(*teedev), GFP_KERNEL);
> > [..]
> > > +	rc = misc_register(&teedev->miscdev);
> > [..]
> > > +void tee_unregister(struct tee_device *teedev)
> > > +{
> > [..]
> > > +	misc_deregister(&teedev->miscdev);
> > > +}
> > [..]
> > >+static int optee_remove(struct platform_device *pdev)
> > >+{
> > >+       tee_unregister(optee->teedev);
> > 
> > Isn't that a potential use after free? AFAIK misc_deregister does not
> > guarentee the miscdev will no longer be accessed after it returns, and
> > the devm will free it after optee_remove returns.
> > 
> > Memory backing a stuct device needs to be freed via the release
> > function.
> 
> Out of interest, which struct device are you talking about here?

Sorry, I was imprecise. In the first paragraph I ment 'miscdev' to
refer to the entire thing, struct tee_device, struct misc_device, the
driver allocations, etc.

So, the first issue is the use-after-free via ioctl() touching struct
tee_device that you described.

But then we trundle down to:

+ ctx->teedev->desc->ops->get_version(ctx, &vers.spec_version,
+   vers.uuid);

If we kref teedev so it is valid then calling a driver call back after
(or during) it's remove function is very likely to blow up.

Also, in TPM we discovered that adding a sysfs file was very ugly
(impossible?) because without the misc_mtx protection that open has,
getting a locked tee_device in the sysfs callback is difficult.

With TPM, we ended up trying lots of options for fixing struct
misc_device in the tpm core, which is handling multiple sub drivers,
and basically gave up. Gave each struct tpm_device an embedded struct
device like Greg suggested here. Then the tpm core is working with the
APIs, not struggling against them.

But this is not a user-space invisible change, so better to do it right
from day 1 ..

We followed rtc as an example of how to create a mid-layer that
exports it's own register function, with char dev and sysfs
components. It seems properly implemented, and has elegant solutions
to these problems (like ops):
 - Don't mess with modules, use 'ops' and set 'ops' to null when the
   driver removes. The driver core will keep the driver module around
   for you bettwen the probe/remove calls. Setting ops = NULL ensures driver
   module code cannot be called after remove.
 - Use locking for 'ops' to serialize driver callbacks with driver removal
 - Embed a struct device/etc in the struct tee_device and use the release
   function to deallocate struct tee_device. All callbacks from the
   driver/char/sysfs core can now use container_of on something that
   is already holds the right kref.
 - Consider an alloc/register pattern as we use now in TPM. This has proven
   smart for TPM as it allows:
       alloc tee_device + init struct device, etc
       driver setup
       core library helper calls for setup/etc
       driver register + char dev publish

It appeared to me this driver was copying TPM's old architecture,
which is very much known to be broken.

Jason