Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

Dave Martin
2019-05-23T10:43:07Z

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