Re: [PATCH v2 00/14] x86: Add support for Clang CFI

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Aug 25, 2021 at 08:49:36AM -0700, Sami Tolvanen wrote:
> On Tue, Aug 24, 2021 at 12:47 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, Aug 23, 2021 at 10:13:04AM -0700, Sami Tolvanen wrote:
> > > This series adds support for Clang's Control-Flow Integrity (CFI)
> > > checking to x86_64. With CFI, the compiler injects a runtime
> > > check before each indirect function call to ensure the target is
> > > a valid function with the correct static type. This restricts
> > > possible call targets and makes it more difficult for an attacker
> > > to exploit bugs that allow the modification of stored function
> > > pointers. For more details, see:
> >
> > If I understand this right; tp_stub_func() in kernel/tracepoint.c
> > violates this (as would much of the HAVE_STATIC_CALL=n code, luckily
> > that is not a valid x86_64 configuration).
> >
> > Specifically, we assign &tp_stub_func to tracepoint_func::func, but that
> > function pointer is only ever indirectly called when cast to the
> > tracepoint prototype:
> >
> >   ((void(*)(void *, proto))(it_func))(__data, args);
> >
> > (see DEFINE_TRACE_FN() in linux/tracepoint.h)
> >
> > This means the indirect function type and the target function type
> > mismatch, resulting in that runtime check you added to trigger.
> 
> Thanks for pointing this out. Yes, that would clearly trip CFI.
> 
> Any concerns about just writing a magic value to the slot instead of
> pointing it to a stub function, and checking for it before the call?

Performance :-) that compare is going to be useless roughly 100% of the
time.

> > Hitting tp_stub_func() at runtime is exceedingly rare, but possible.
> >
> > I realize this is strictly UB per C, but realistically any CDECL ABI
> > requires that any function with arbitrary signature:
> >
> >   void foo(...)
> >   {
> >   }
> >
> > translates to the exact same code. Specifically on x86-64, the super
> > impressive:
> >
> > foo:
> >         RET
> >
> > And as such this works just fine. Except now you wrecked it.
> 
> Sure. Another option is to disable CFI for the functions that perform
> the call, but I would rather avoid that whenever possible.

Is there no means of teaching the compiler about these magical
functions? There's only two possible stubs:

  void foo(...)
  {
  }

and

  unsigned long bar(...)
  {
	return 0;
  }

Both exist in the kernel. We can easily give them a special function
attribute to call them out.