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

[H.J. Lu]

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.


-- 
H.J.