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

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Date:   Wed, 25 Aug 2021 08:49:36 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <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,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI

On Tue, Aug 24, 2021 at 12:47 PM Peter Zijlstra <peterz@...radead.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?

> 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.

Sami