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
