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*. (Also, shouldn't the vDSO itself be marked as supporting CET?)
