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

[Catalin Marinas]

On Thu, May 23, 2019 at 08:44:12AM -0700, enh wrote:
> On Thu, May 23, 2019 at 7:45 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > On Wed, May 22, 2019 at 01:47:36PM -0700, Kees Cook wrote:
> > > For userspace, how would a future binary choose TBI over MTE? If it's
> > > a library issue, we can't use an ELF bit, since the choice may be
> > > "late" after ELF load (this implies the need for a prctl().) If it's
> > > binary-only ("built with HWKASan") then an ELF bit seems sufficient.
> > > And without the marking, I'd expect the kernel to enforce MTE when
> > > there are high bits.
> >
> > The current plan is that a future binary issues a prctl(), after
> > checking the HWCAP_MTE bit (as I replied to Elliot, the MTE instructions
> > are not in the current NOP space). I'd expect this to be done by the
> > libc or dynamic loader under the assumption that the binaries it loads
> > do _not_ use the top pointer byte for anything else.
> 
> yeah, it sounds like to support hwasan and MTE, the dynamic linker
> will need to not use either itself.
> 
> > With hwasan compiled objects this gets more confusing (any ELF note
> > to identify them?).
> 
> no, at the moment code that wants to know checks for the presence of
> __hwasan_init. (and bionic doesn't actually look at any ELF notes
> right now.) but we can always add something if we need to.

It's a userspace decision to make. In the kernel, we are proposing that
bionic calls a prctl() to enable MTE explicitly. It could first check
for the presence of __hwasan_init.

-- 
Catalin