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

[Alice Ryhl]

On Mon, Dec 9, 2024 at 2:13 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Dec 09, 2024 at 01:53:42PM +0100, Alice Ryhl wrote:
> > On Mon, Dec 9, 2024 at 1:08 PM Greg Kroah-Hartman
> > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx> 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 :)
> >
> > I grepped for misc_deregister and the first driver I looked at is
> > drivers/misc/bcm-vk which seems to allow dynamic deregistration if the
> > pci device is removed.
>
> Ah, yeah, that's going to get messy and will be a problem if someone has
> the file open then.
>
> > Another tricky path is error cleanup in its probe function.
> > Technically, if probe fails after registering the misc device, there's
> > a brief moment where you could open the miscdevice before it gets
> > removed in the cleanup path, which seems to me that it could lead to
> > UAF?
> >
> > Or is there something I'm missing?
>
> Nope, that too is a window of a problem, luckily you "should" only
> register the misc device after you know the device is safe to use as
> once it is registered, it could be used so it "should" be the last thing
> you do in probe.
>
> So yes, you are right, and we do know about these issues (again see the
> talk I mentioned and some previous ones for many years at plumbers
> conferences by different people.)  It's just up to someone to do the
> work to fix them.
>
> If you think we can prevent the race in the rust side, wonderful, I'm
> all for that being a valid fix.

The current patch prevents the race by only allowing access to the
`struct miscdevice` in fops->open(). That's safe since
`file->f_op->open` runs with `misc_mtx` held. Do we really need the
miscdevice to stay alive for longer? You can already take a refcount
on `this_device` if you want to keep the device alive for longer for
dev_* printing purposes, but it seems like that is the only field you
really need from the `struct miscdevice` past fops->open()?

Alice