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

[Peter Zijlstra <peterz@infradead.org>]

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.

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.