Re: [PATCH v2 2/2] rust: miscdevice: access the `struct miscdevice` from fops->open()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Dec 09, 2024 at 01:00:05PM +0100, Alice Ryhl wrote:
> On Mon, Dec 9, 2024 at 12:53 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Mon, Dec 09, 2024 at 12:38:32PM +0100, Alice Ryhl wrote:
> > > On Mon, Dec 9, 2024 at 12:10 PM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Mon, Dec 09, 2024 at 11:50:57AM +0100, Alice Ryhl wrote:
> > > > > On Mon, Dec 9, 2024 at 9:48 AM Greg Kroah-Hartman
> > > > > <gregkh@linuxfoundation.org> wrote:
> > > > > >
> > > > > > On Mon, Dec 09, 2024 at 07:27:47AM +0000, Alice Ryhl wrote:
> > > > > > > Providing access to the underlying `struct miscdevice` is useful for
> > > > > > > various reasons. For example, this allows you access the miscdevice's
> > > > > > > internal `struct device` for use with the `dev_*` printing macros.
> > > > > > >
> > > > > > > Note that since the underlying `struct miscdevice` could get freed at
> > > > > > > any point after the fops->open() call, only the open call is given
> > > > > > > access to it. To print from other calls, they should take a refcount on
> > > > > > > the device to keep it alive.
> > > > > >
> > > > > > The lifespan of the miscdevice is at least from open until close, so
> > > > > > it's safe for at least then (i.e. read/write/ioctl/etc.)
> > > > >
> > > > > How is that enforced? What happens if I call misc_deregister while
> > > > > there are open fds?
> > > >
> > > > You shouldn't be able to do that as the code that would be calling
> > > > misc_deregister() (i.e. in a module unload path) would not work because
> > > > the module reference count is incremented at this point in time due to
> > > > the file operation module reference.
> > >
> > > Oh .. so misc_deregister must only be called when the module is being unloaded?
> >
> > Traditionally yes, that's when it is called.  Do you see it happening in
> > any other place in the kernel today?
> 
> I had not looked, but I know that Binder allows dynamically creating
> and removing its devices at runtime. It happens to be the case that
> this is only supported when binderfs is used, which is when it doesn't
> use miscdevice, so technically Binder does not call misc_deregister()
> outside of module unload, but following its example it's not hard to
> imagine that such removals could happen.

That's why those are files and not misc devices :)

> > > > Yeah, it's a horrid hack, and one day we will put "real" revoke logic in
> > > > here to detach the misc device from the file operations if this were to
> > > > happen.  It's a very very common anti-pattern that many subsystems have
> > > > that is a bug that we all have been talking about for a very very long
> > > > time.  Wolfram even has a plan for how to fix it all up (see his Japan
> > > > LinuxCon talk from 2 years ago), but I don't think anyone is doing the
> > > > work on it :(
> > > >
> > > > The media and drm layers have internal hacks/work-arounds to try to
> > > > handle this issue, but luckily for us, the odds of a misc device being
> > > > dynamically removed from the system is pretty low.
> > > >
> > > > Once / if ever, we get the revoke type logic implemented, then we can
> > > > apply that to the misc device code and follow it through to the rust
> > > > side if needed.
> > >
> > > If dynamically deregistering is not safe, then we need to change the
> > > Rust abstractions to prevent it.
> >
> > Dynamically deregistering is not unsafe, it's just that I don't think
> > you will physically ever have the misc_deregister() path called if a
> > file handle is open.  Same should be the case for rust code, it should
> > "just work" without any extra code to do so.
> 
> Well, if I give files access to the struct miscdevice in all fops
> hooks, then deregistering does become unsafe since accessing it in an
> ioctl after deregistering would be a UAF. I'd like to prevent the user
> from doing that.

I don't think that the deregister would succeed in the vfs layer if an
open file reference was currently held, but I haven't tried that in a
long time.

If you can come up with a way to prevent that, wonderful, but I wouldn't
worry too much as again, this "should not" happen due to the file
reference count, and if it does, it's a major logic error on the
driver's part, just like we have today in C.

thanks,

greg k-h