Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

[Luc Van Oostenryck]

On Thu, Jun 28, 2018 at 11:27:42AM +0100, Catalin Marinas wrote:
> On Thu, Jun 28, 2018 at 08:17:59AM +0200, Luc Van Oostenryck wrote:
> > On Wed, Jun 27, 2018 at 06:17:58PM +0100, Catalin Marinas wrote:
> > > sparse is indeed an option. The current implementation doesn't warn on
> > > an explicit cast from (void __user *) to (unsigned long) since that's a
> > > valid thing in the kernel. I couldn't figure out if there's any other
> > > __attribute__ that could be used to warn of such conversion.
> > 
> > sparse doesn't have such attribute but would an new option that would warn
> > on such cast be a solution for your case?
> 
> I can't tell for sure whether such sparse option would be the full
> solution but detecting explicit __user pointer casts to long is a good
> starting point. So far this patchset pretty much relies on detecting
> a syscall failure and trying to figure out why, patching the kernel. It
> doesn't really scale.

OK, I'll add such an option this evening.
 
> As a side note, we have cases in the user-kernel ABI where the user
> address type is "unsigned long": mmap() and friends. My feedback on an
> early version of this patchset was to always require untagged pointers
> coming from user space on such syscalls, so no need for explicit
> untagging.

Mmmm yes.
I tend to favor a sort of opposite approach. When we have an address
that must not be dereferenced as-such (and sometimes when the address
can be from both __user & __kernel space) I prefer to use a ulong
which will force the use of the required operation before being
able to do any sort of dereferencing and this won't need horrible
casts with __force (it, of course, all depends on the full context).

-- Luc