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

[Yu-cheng Yu]

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.ÂÂI 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. ÂIf the loader has CET in its ELF
header, it is executed with CET on. ÂThe loader will turn off CET if
the application being loaded does not support it (in the ELF header).
ÂThe loader calls ARCH_CET_LOCK before passing to the application. ÂBut
how do we handle dlopen?

> > 
> > Ptrace gets new APIs to turn CET on and off and to lock and unlock
> > it.ÂÂIf 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?

Yu-cheng