Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack

[Andy Lutomirski <luto@amacapital.net>]

> On Jun 19, 2018, at 10:07 AM, Kees Cook <keescook@chromium.org> wrote:
> 
>> On Tue, Jun 19, 2018 at 9:59 AM, Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>> On Tue, 2018-06-19 at 09:44 -0700, Kees Cook wrote:
>>> On Tue, Jun 19, 2018 at 7:50 AM, Andy Lutomirski <luto@amacapital.net
>>>> wrote:
>>>> 
>>>>> 
>>>>> On Jun 18, 2018, at 5:52 PM, Kees Cook <keescook@chromium.org>
>>>>> wrote:
>>>>> Following Linus's request for "slow introduction" of new security
>>>>> features, likely the best approach is to default to "relaxed"
>>>>> (with a
>>>>> warning about down-grades), and allow distros/end-users to pick
>>>>> "forced" if they know their libraries are all CET-enabled.
>>>> I still don’t get what “relaxed” is for.  I think the right design
>>>> is:
>>>> 
>>>> Processes start with CET on or off depending on the ELF note, but
>>>> they start with CET unlocked no matter what. They can freely switch
>>>> CET on and off (subject to being clever enough not to crash if they
>>>> turn it on and then return right off the end of the shadow stack)
>>>> until they call ARCH_CET_LOCK.
>>> I'm fine with this. I'd expect modern loaders to just turn on CET and
>>> ARCH_CET_LOCK immediately and be done with it. :P
>> 
>> This is the current implementation.  If the loader has CET in its ELF
>> header, it is executed with CET on.  The loader will turn off CET if
>> the application being loaded does not support it (in the ELF header).
>> The loader calls ARCH_CET_LOCK before passing to the application.  But
>> how do we handle dlopen?
> 
> I thought CET_LOCK would not get set in "relaxed" mode, due to dlopen
> usage, and that would be the WARN case. People without dlopen concerns
> can boot with "enforced" mode? If a system builder knows there are no
> legacy dlopens they build with enforced enabled, etc.

I think we’re getting ahead of ourselves. dlopen() of a non-CET-aware library in a CET process is distinctly non-trivial, especially in a multithreaded process. I think getting it right will require *userspace* support.  It certainly needs ld.so to issue to arch_prctl at a bare minimum. So I see no point to a kernel-supplied “relaxed” mode. I think there may be demand for a ld.so relaxed mode, but it will have nothing to do with boot options.

It’s potentially helpful to add an arch_prctl that turns CET off for all threads, but only if unlocked. It would obviously be one hell of a gadget.

> 
>>>> Ptrace gets new APIs to turn CET on and off and to lock and unlock
>>>> it.  If an attacker finds a “ptrace me and turn off CET” gadget,
>>>> then they might as well just do “ptrace me and write shell code”
>>>> instead. It’s basically the same gadget. Keep in mind that the
>>>> actual sequence of syscalls to do this is incredibly complicated.
>>> Right -- if an attacker can control ptrace of the target, we're way
>>> past CET. The only concern I have, though, is taking advantage of
>>> expected ptracing. For example: browsers tend to have crash handlers
>>> that launch a ptracer. If ptracing disabled CET for all threads, this
>>> won't by safe: an attacker just gains control in two threads, crashes
>>> one to get the ptracer to attach, which disables CET in the other
>>> thread and the attacker continues ROP as normal. As long as the
>>> ptrace
>>> disabling is thread-specific, I think this will be okay.
>> 
>> If ptrace can turn CET on/off and it is thread-specific, do we still
>> need ptrace lock/unlock?

Let me clarify. I don’t think ptrace() should have any automatic effect on CET. I think there should be an explicit way to ask ptrace to twiddle CET, and it should probably apply per thread.

> 
> Does it provide anything beyond what PR_DUMPABLE does?

What do you mean?


> 
> -Kees
> 
> -- 
> Kees Cook
> Pixel Security