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

[Peter Zijlstra]

On Tue, Feb 07, 2017 at 11:10:12AM +0000, Mark Rutland wrote:
> 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? :)

Right something along those lines. (a) will need GCC help, and (b) would
be kernel-arch specific. So this isn't something we can quickly do.