RE: [PATCH 05/15] x86: Implement function_nocfi

[David Laight]

From: Kees Cook
> Sent: 16 April 2021 23:28
> 
> On Fri, Apr 16, 2021 at 03:06:17PM -0700, Andy Lutomirski wrote:
> > On Fri, Apr 16, 2021 at 3:03 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
> > >
> > > On Fri, Apr 16, 2021 at 02:49:23PM -0700, Sami Tolvanen wrote:
> > > > __nocfi only disables CFI checking in a function, the compiler still
> > > > changes function addresses to point to the CFI jump table, which is
> > > > why we need function_nocfi().
> > >
> > > So call it __func_addr() or get_function_addr() or so, so that at least
> > > it is clear what this does.
> > >
> >
> > This seems backwards to me.  If I do:
> >
> > extern void foo(some signature);
> >
> > then I would, perhaps naively, expect foo to be the actual symbol that
> > gets called
> 
> Yes.
> 
> > and for the ABI to be changed to do the CFI checks.
> 
> Uh, no? There's no ABI change -- indirect calls are changed to do the
> checking.
> 
> > The
> > foo symbol would point to whatever magic is needed.
> 
> No, the symbol points to the jump table entry. Direct calls get minimal
> overhead and indirect calls can add the "is this function in the right
> table" checking.


Isn't this a bit like one of the PPC? ABI where function addresses
aren't (always) the entry point.
IIRC is causes all sorts of obscure grief.

I'd also like to know how indirect calls are actually expected to work.
The whole idea is that the potential targets might be in a kernel module
that is loaded at run time.

Scanning a list of possible targets has to be a bad design decision.

If you are trying to check that the called function has the right
prototype, stashing a hash of the prototype (or a known random number)
before the entry point would give most of the benefits without most
of the costs.
The linker could be taught to do the same test (much like name mangling
but without the crap user experience).

That scheme only has the downside of a data cache miss and (hopefully)
statically predicted correct branch (hmm - except you don't want to
speculatively execute the wrong function) and polluting the data cache
with code.

This all seems like a ploy to force people to buy faster cpus.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)