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 :) > 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. --Andy
