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

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Date:   Tue, 24 Aug 2021 21:46:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI

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.