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. >> When the next exec() is called, CET >> feature is turned on/off based on the values set by ARCH_CET_EXEC. > > And why do we need ARCH_CET_EXEC? > > For background, I really really dislike adding new state that persists > across exec(). It's nice to get as close to a clean slate as possible > after exec() so that programs can run in a predictable environment. > exec() is also a security boundary, and anything a task can do to > affect itself after exec() needs to have its security implications > considered very carefully. (As a trivial example, you should not be > able to use cetcmd ... sudo [malicious options here] to cause sudo to > run with CET off and then try to exploit it via the malicious options. > > If a shutoff is needed for testing, how about teaching ld.so to parse > LD_CET=no or similar and protect it the same way as LD_PRELOAD is > protected. Or just do LD_PRELOAD=/lib/libdoesntsupportcet.so. > I will take a look. -- H.J.
