Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION

[Mark Rutland]

On Tue, Feb 07, 2017 at 09:34:05AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 06, 2017 at 08:54:38AM -0800, Kees Cook wrote:
> > >
> > > Like I wrote, ideally we'd end up using something like the x86 exception
> > > table with a custom handler. Just no idea how to pull that off without
> > > doing a full blown arch specific implementation, so I didn't go there
> > > quite yet.
> > 
> > I haven't spent much time looking at the extable stuff. (Though
> > coincidentally, I was poking at it for x86's test_nx stuff...) I
> > thought there was a way to build arch-agnostic extables already?
> > kernel/extable.c is unconditionally built-in, for example.
> 
> That doesn't seem to be of much use. It only contains section sort and
> search functions.
> 
> Another problem for generic code would be to figure out what register
> the relevant variable would live in at the time of exception. Here its
> 'obviously' EAX because that's what cmpxchg requires, but in generic
> you'd need a means of querying GCC's register allocator at the exception
> point and somehow using that information for the generation of the
> exception handler.

I think we only need two arch-specific primitives:
(a) mangle a GCC assigned register into an idx stored in the extable
(b) take said index, and grab the relevant register from pt_regs

Then you can have a BUG_VALUE(v, ...), where we use an input "r" (val),
and mangle that into the idx in the extable. In the common case, I'd
hope GCC would leave the register in-place from the cmpxchg.

... or have I misundertood? :)

Thanks,
Mark.