[PATCH 0/6] arm64: untag user pointers passed to the kernel

[Catalin Marinas]

On Wed, Apr 25, 2018 at 04:45:37PM +0200, Andrey Konovalov wrote:
> On Thu, Apr 19, 2018 at 11:33 AM, Kirill A. Shutemov
> <kirill at shutemov.name> wrote:
> > On Wed, Apr 18, 2018 at 08:53:09PM +0200, Andrey Konovalov wrote:
> >> arm64 has a feature called Top Byte Ignore, which allows to embed pointer
> >> tags into the top byte of each pointer. Userspace programs (such as
> >> HWASan, a memory debugging tool [1]) might use this feature and pass
> >> tagged user pointers to the kernel through syscalls or other interfaces.
> >>
> >> This patch makes a few of the kernel interfaces accept tagged user
> >> pointers. The kernel is already able to handle user faults with tagged
> >> pointers and has the untagged_addr macro, which this patchset reuses.
> >>
> >> We're not trying to cover all possible ways the kernel accepts user
> >> pointers in one patchset, so this one should be considered as a start.
> >
> > How many changes do you anticipate?
> >
> > This patchset looks small and reasonable, but I see a potential to become a
> > boilerplate. Would we need to change every driver which implements ioctl()
> > to strip these bits?
> 
> I've replied to somewhat similar question in one of the previous
> versions of the patchset.
> 
> """
> There are two different approaches to untagging the user pointers that I see:
> 
> 1. Untag user pointers right after they are passed to the kernel.
> 
> While this might be possible for pointers that are passed to syscalls
> as arguments (Catalin's "hack"), this leaves user pointers, that are
> embedded into for example structs that are passed to the kernel. Since
> there's no specification of the interface between user space and the
> kernel, different kernel parts handle user pointers differently and I
> don't see an easy way to cover them all.
> 
> 2. Untag user pointers where they are used in the kernel.
> 
> Although there's no specification on the interface between the user
> space and the kernel, the kernel still has to use one of a few
> specific ways to access user data (copy_from_user, etc.). So the idea
> here is to add untagging into them. This patchset mostly takes this
> approach (with the exception of memory subsystem syscalls).
> 
> If there's a better approach, I'm open to suggestions.
> """
> 
> So if we go with the first way, we'll need to go through every syscall
> and ioctl handler, which doesn't seem feasible.

I agree with you that (1) isn't feasible. My hack is sufficient for the
pointer arguments but doesn't help with pointers in user structures
passed to the kernel.

Now, since the hardware allows access to user pointers with non-zero top
8-bit, the kernel uaccess routines can also use such pointers directly.
What's needed, as per your patches, is the access_ok() macro and
whatever ends up calling find_vma() (at a first look, there may be other
cases). I don't think drivers need changing as long as the in-kernel API
they use performs the untagging (e.g. get_user_pages()).

-- 
Catalin