On Wed, May 22, 2019 at 09:20:52PM -0300, Jason Gunthorpe wrote: > On Wed, May 22, 2019 at 02:49:28PM +0100, Dave Martin wrote: > > On Tue, May 21, 2019 at 03:48:56PM -0300, Jason Gunthorpe wrote: > > > On Fri, May 17, 2019 at 03:49:31PM +0100, Catalin Marinas wrote: > > > > > > > The tagged pointers (whether hwasan or MTE) should ideally be a > > > > transparent feature for the application writer but I don't think we can > > > > solve it entirely and make it seamless for the multitude of ioctls(). > > > > I'd say you only opt in to such feature if you know what you are doing > > > > and the user code takes care of specific cases like ioctl(), hence the > > > > prctl() proposal even for the hwasan. > > > > > > I'm not sure such a dire view is warrented.. > > > > > > The ioctl situation is not so bad, other than a few special cases, > > > most drivers just take a 'void __user *' and pass it as an argument to > > > some function that accepts a 'void __user *'. sparse et al verify > > > this. > > > > > > As long as the core functions do the right thing the drivers will be > > > OK. > > > > > > The only place things get dicy is if someone casts to unsigned long > > > (ie for vma work) but I think that reflects that our driver facing > > > APIs for VMAs are compatible with static analysis (ie I have no > > > earthly idea why get_user_pages() accepts an unsigned long), not that > > > this is too hard. > > > > If multiple people will care about this, perhaps we should try to > > annotate types more explicitly in SYSCALL_DEFINEx() and ABI data > > structures. > > > > For example, we could have a couple of mutually exclusive modifiers > > > > T __object * > > T __vaddr * (or U __vaddr) > > > > In the first case the pointer points to an object (in the C sense) > > that the call may dereference but not use for any other purpose. > > How would you use these two differently? > > So far the kernel has worked that __user should tag any pointer that > is from userspace and then you can't do anything with it until you > transform it into a kernel something Ultimately it would be good to disallow casting __object pointers execpt to compatible __object pointer types, and to make get_user etc. demand __object. __vaddr pointers / addresses would be freely castable, but not to __object and so would not be dereferenceable even indirectly. Or that's the general idea. Figuring out a sane set of rules that we could actually check / enforce would require a bit of work. (Whether the __vaddr base type is a pointer or an integer type is probably moot, due to the restrictions we would place on the use of these anyway.) > > to tell static analysers the real type of pointers smuggled through > > UAPI disguised as other types (*cough* KVM, etc.) > > Yes, that would help alot, we often have to pass pointers through a > u64 in the uAPI, and there is no static checker support to make sure > they are run through the u64_to_user_ptr() helper. Agreed. Cheers ---Dave
