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

Jason Gunthorpe
2019-05-23T00:20:56Z

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

> 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.

Jason