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

[Kees Cook]

On Tue, Jun 19, 2018 at 9:59 AM, Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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@xxxxxxxxxxxxxx
>> > wrote:
>> >
>> > >
>> > > On Jun 18, 2018, at 5:52 PM, Kees Cook <keescook@xxxxxxxxxxxx>
>> > > 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 donât get what ârelaxedâ 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.

>> > Ptrace gets new APIs to turn CET on and off and to lock and unlock
>> > it.  If an attacker finds a âptrace me and turn off CETâ gadget,
>> > then they might as well just do âptrace me and write shell codeâ
>> > instead. Itâs 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?

Does it provide anything beyond what PR_DUMPABLE does?

-Kees

-- 
Kees Cook
Pixel Security