lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Wed, 25 Aug 2021 08:49:36 -0700
From:   Sami Tolvanen <>
To:     Peter Zijlstra <>
Cc:     X86 ML <>, Kees Cook <>,
        Josh Poimboeuf <>,
        Nathan Chancellor <>,
        Nick Desaulniers <>,
        Sedat Dilek <>,,
        LKML <>,
        clang-built-linux <>,
        Steven Rostedt <>
Subject: Re: [PATCH v2 00/14] x86: Add support for Clang CFI

On Tue, Aug 24, 2021 at 12:47 PM Peter Zijlstra <> 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.


Powered by blists - more mailing lists