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

[Kees Cook]

On Mon, Jun 18, 2018 at 3:03 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> On Tue, Jun 12, 2018 at 12:34 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>>
>> On Tue, Jun 12, 2018 at 11:59 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> > On Tue, 12 Jun 2018, H.J. Lu wrote:
>> >> On Tue, Jun 12, 2018 at 9:34 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> >> > On Tue, Jun 12, 2018 at 9:05 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>> >> >> On Tue, Jun 12, 2018 at 9:01 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> >> >> > On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>> >> >> >> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> >> >> >> > That works for stuff which loads all libraries at start time, but what
>> >> >> >> > happens if the program uses dlopen() later on? If CET is force locked and
>> >> >> >> > the library is not CET enabled, it will fail.
>> >> >> >>
>> >> >> >> That is to prevent disabling CET by dlopening a legacy shared library.
>> >> >> >>
>> >> >> >> > I don't see the point of trying to support CET by magic. It adds complexity
>> >> >> >> > and you'll never be able to handle all corner cases correctly. dlopen() is
>> >> >> >> > not even a corner case.
>> >> >> >>
>> >> >> >> That is a price we pay for security.  To enable CET, especially shadow
>> >> >> >> shack, the program and all of shared libraries it uses should be CET
>> >> >> >> enabled.  Most of programs can be enabled with CET by compiling them
>> >> >> >> with -fcf-protection.
>> >> >> >
>> >> >> > If you charge too high a price for security, people may turn it off.
>> >> >> > I think we're going to need a mode where a program says "I want to use
>> >> >> > the CET, but turn it off if I dlopen an unsupported library".  There
>> >> >> > are programs that load binary-only plugins.
>> >> >>
>> >> >> You can do
>> >> >>
>> >> >> # export GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK
>> >> >>
>> >> >> which turns off shadow stack.
>> >> >>
>> >> >
>> >> > Which exactly illustrates my point.  By making your security story too
>> >> > absolute, you'll force people to turn it off when they don't need to.
>> >> > If I'm using a fully CET-ified distro and I'm using a CET-aware
>> >> > program that loads binary plugins, and I may or may not have an old
>> >> > (binary-only, perhaps) plugin that doesn't support CET, then the
>> >> > behavior I want is for CET to be on until I dlopen() a program that
>> >> > doesn't support it.  Unless there's some ABI reason why that can't be
>> >> > done, but I don't think there is.
>> >>
>> >> We can make it opt-in via GLIBC_TUNABLES.  But by default, the legacy
>> >> shared object is disallowed when CET is enabled.
>> >
>> > That's a bad idea. Stuff has launchers which people might not be able to
>> > change. So they will simply turn of CET completely or it makes them hack
>> > horrible crap into init, e.g. the above export.
>> >
>> > Give them sane kernel options:
>> >
>> >      cet = off, relaxed, forced
>> >
>> > where relaxed allows to run binary plugins. Then let dlopen() call into the
>> > kernel with the filepath of the library to check for CET and that will tell
>> > you whether its ok or or not and do the necessary magic in the kernel when
>> > CET has to be disabled due to a !CET library/application.
>> >
>> > That's also making the whole thing independent of magic glibc environment
>> > options and allows it to be used all over the place in the same way.
>>
>> This is very similar to our ARCH_CET_EXEC proposal which controls how
>> CET should be enforced.   But Andy thinks it is a bad idea.
>
> I do think it's a bad idea to have a new piece of state that survives
> across exec().  It's going to have nasty usability problems and nasty
> security problems.
>
> We may need a mode by which glibc can turn CET *back off* even after a
> program had it on if it dlopens() an old binary.  Or maybe there won't
> be demand.  I can certainly understand why the CET_LOCK feature is
> there, although I think we need a way to override it using something
> like ptrace().  I'm not convinced that CET_LOCK is really needed, but
> someone who understand the thread model should chime in.
>
> Kees, do you know anyone who has a good enough understanding of
> usermode exploits and how they'll interact with CET?

Adding Florian to CC, but if something gets CET enabled, it really
shouldn't have a way to turn it off. If there's a way to turn it off,
all the ROP research will suddenly turn to exactly one gadget before
doing the rest of the ROP: turning off CET. Right now ROP is: use
stack-pivot gadget, do everything else. Allowed CET to turn off will
just add one step: use CET-off gadget, use stack-pivot gadget, do
everything else. :P

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.

-Kees

-- 
Kees Cook
Pixel Security