On Thu, Jun 7, 2018 at 9:38 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > On Thu, Jun 7, 2018 at 9:10 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: >> >> On Thu, Jun 7, 2018 at 4:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> > On Thu, Jun 7, 2018 at 3:02 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: >> >> >> >> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >> >> > On Thu, Jun 7, 2018 at 1:33 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: >> >> >> >> >> >> On Thu, 2018-06-07 at 11:48 -0700, Andy Lutomirski wrote: >> >> >> > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote: >> >> >> > > >> >> >> > > The following operations are provided. >> >> >> > > >> >> >> > > ARCH_CET_STATUS: >> >> >> > > return the current CET status >> >> >> > > >> >> >> > > ARCH_CET_DISABLE: >> >> >> > > disable CET features >> >> >> > > >> >> >> > > ARCH_CET_LOCK: >> >> >> > > lock out CET features >> >> >> > > >> >> >> > > ARCH_CET_EXEC: >> >> >> > > set CET features for exec() >> >> >> > > >> >> >> > > ARCH_CET_ALLOC_SHSTK: >> >> >> > > allocate a new shadow stack >> >> >> > > >> >> >> > > ARCH_CET_PUSH_SHSTK: >> >> >> > > put a return address on shadow stack >> >> >> > > >> >> >> > > ARCH_CET_ALLOC_SHSTK and ARCH_CET_PUSH_SHSTK are intended only for >> >> >> > > the implementation of GLIBC ucontext related APIs. >> >> >> > >> >> >> > Please document exactly what these all do and why. I don't understand >> >> >> > what purpose ARCH_CET_LOCK and ARCH_CET_EXEC serve. CET is opt in for >> >> >> > each ELF program, so I think there should be no need for a magic >> >> >> > override. >> >> >> >> >> >> CET is initially enabled if the loader has CET capability. Then the >> >> >> loader decides if the application can run with CET. If the application >> >> >> cannot run with CET (e.g. a dependent library does not have CET), then >> >> >> the loader turns off CET before passing to the application. When the >> >> >> loader is done, it locks out CET and the feature cannot be turned off >> >> >> anymore until the next exec() call. >> >> > >> >> > 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. >> > >> > Yeah, I got that. No one has explained *why*. >> >> It is to prevent malicious code from disabling CET. >> > > By the time malicious code issue its own syscalls, you've already lost > the battle. I could probably be convinced that a lock-CET-on feature > that applies *only* to the calling thread and is not inherited by > clone() is a decent idea, but I'd want to see someone who understands > the state of the art in exploit design justify it. You're also going > to need to figure out how to make CRIU work if you allow locking CET > on. > > A priori, I think we should just not provide a lock mechanism. We need a door for CET. But it is a very bad idea to leave it open all the time. I don't know much about CRIU, If it is Checkpoint/Restore In Userspace. Can you free any application with AVX512 on AVX512 machine and restore it on non-AVX512 machine? >> > (Also, shouldn't the vDSO itself be marked as supporting CET?) >> >> No. vDSO is loaded by kernel. vDSO in CET kernel is CET >> compatible. >> > > I think the vDSO should do its best to act like a real DSO. That > means that, if the vDSO supports CET, it should advertise support for > CET using the Linux ABI. Since you're going to require GCC 8 anyway, > this should be a single line of code in the Makefile. Sure. A couple lines. -- H.J.
