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

Dave Martin
2019-05-22T13:49:40Z

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.

In the latter case the pointer (or other type) is a virtual address
that the call does not dereference but my do other things with.

Also

U __really(T)

to tell static analysers the real type of pointers smuggled through
UAPI disguised as other types (*cough* KVM, etc.)

We could gradually make sparse more strict about the presence of
annotations and allowed conversions, add get/put_user() variants
that demand explicit annotation, etc.

find_vma() wouldn't work with a __object pointer, for example. A
get_user_pages_for_dereference() might be needed for __object pointers
(embodying a promise from the caller that only the object will be
dereferenced within the mapped pages).

Thoughts?

This kind of thing would need widespread buy-in in order to be viable.

Cheers
---Dave