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

[H.J. Lu]

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:
>> >> > On Thu, 7 Jun 2018, H.J. Lu wrote:
>> >> >> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> >> >> > Why is the lockout necessary?  If user code enables CET and tries to
>> >> >> > run code that doesn't support CET, it will crash.  I don't see why we
>> >> >> > need special code in the kernel to prevent a user program from calling
>> >> >> > arch_prctl() and crashing itself.  There are already plenty of ways to
>> >> >> > do that :)
>> >> >>
>> >> >> On CET enabled machine, not all programs nor shared libraries are
>> >> >> CET enabled.  But since ld.so is CET enabled, all programs start
>> >> >> as CET enabled.  ld.so will disable CET if a program or any of its shared
>> >> >> libraries aren't CET enabled.  ld.so will lock up CET once it is done CET
>> >> >> checking so that CET can't no longer be disabled afterwards.
>> >> >
>> >> > 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.

> I'm concerned that the entire concept of locking CET is there to solve
> a security problem that doesn't actually exist.

We don't know that.


-- 
H.J.