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

[H.J. Lu]

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.


-- 
H.J.