Messages in this thread | | | Subject | Re: [PATCH 04/10] x86/cet: Handle thread shadow stack | From | Florian Weimer <> | Date | Fri, 8 Jun 2018 16:53:03 +0200 |
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On 06/07/2018 10:53 PM, Andy Lutomirski wrote: > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer <fweimer@redhat.com> wrote: >> >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote: >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote: >>>> >>>> When fork() specifies CLONE_VM but not CLONE_VFORK, the child >>>> needs a separate program stack and a separate shadow stack. >>>> This patch handles allocation and freeing of the thread shadow >>>> stack. >>> >>> Aha -- you're trying to make this automatic. I'm not convinced this >>> is a good idea. The Linux kernel has a long and storied history of >>> enabling new hardware features in ways that are almost entirely >>> useless for userspace. >>> >>> Florian, do you have any thoughts on how the user/kernel interaction >>> for the shadow stack should work? >> >> I have not looked at this in detail, have not played with the emulator, >> and have not been privy to any discussions before these patches have >> been posted, however … >> >> I believe that we want as little code in userspace for shadow stack >> management as possible. One concern I have is that even with the code >> we arguably need for various kinds of stack unwinding, we might have >> unwittingly built a generic trampoline that leads to full CET bypass. > > I was imagining an API like "allocate a shadow stack for the current > thread, fail if the current thread already has one, and turn on the > shadow stack". glibc would call clone and then call this ABI pretty > much immediately (i.e. before making any calls from which it expects > to return).
Ahh. So you propose not to enable shadow stack enforcement on the new thread even if it is enabled for the current thread? For the cases where CLONE_VM is involved?
It will still need a new assembler wrapper which sets up the shadow stack, and it's probably required to disable signals.
I think it should be reasonable safe and actually implementable. But the benefits are not immediately obvious to me.
> We definitely want strong enough user control that tools like CRIU can > continue to work. I haven't looked at the SDM recently enough to > remember for sure, but I'm reasonably confident that user code can > learn the address of its own shadow stack. If nothing else, CRIU > needs to be able to restore from a context where there's a signal on > the stack and the signal frame contains a shadow stack pointer.
CRIU also needs the shadow stack *contents*, which shouldn't be directly available to the process. So it needs very special interfaces anyway.
Does CRIU implement MPX support?
Thanks, Florian
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