On 5/15/20 2:33 PM, Yu-cheng Yu wrote: > On Fri, 2020-05-15 at 11:39 -0700, Dave Hansen wrote: >> On 5/12/20 4:20 PM, Yu-cheng Yu wrote: >> Can a binary compiled with CET run without CET? > > Yes, but a few details: > > - The shadow stack is transparent to the application. A CET application does > not have anything different from a non-CET application. However, if a CET > application uses any CET instructions (e.g. INCSSP), it must first check if CET > is turned on. > - If an application is compiled for IBT, the compiler inserts ENDBRs at branch > targets. These are nops if IBT is not on. I appreciate the detailed response, but it wasn't quite what I was asking. Let's ignore IBT for now and just talk about shadow stacks. An app compiled with the new ELF flags and running on a CET-enabled kernel and CPU will start off with shadow stacks allocated and enabled, right? It can turn its shadow stack off per-thread with the new prctl. But, otherwise, it's stuck, the only way to turn shadow stacks off at startup would be editing the binary. Basically, if there ends up being a bug in an app that violates the shadow stack rules, the app is broken, period. The only recourse is to have the kernel disable CET and reboot. Is that right? >> Can a binary compiled without CET run CET-enabled code? > > Partially yes, but in reality somewhat difficult. ... > - If a not-CET application does fork(), and the child wants to turn on CET, it > would be difficult to manage the stack frames, unless the child knows what is is > doing. It might be hard to do, but it is possible with the patches you posted? I think you're saying that the CET-enabled binary would do arch_setup_elf_property() when it was first exec()'d. Later, it could use the new prctl(ARCH_X86_CET_DISABLE) to disable its shadow stack, then fork() and the child would not be using CET. Right? What is ARCH_X86_CET_DISABLE used for, anyway? > The JIT examples I mentioned previously run with CET enabled from the > beginning. Do you have a reason to do this? In other words, if the JIT code > needs CET, the app could have started with CET in the first place. Let's say I have a JIT'd sandbox. I want the sandbox to be CET-protected, but the JIT engine itself not to be. > - If you are asking about dlopen(), the library will have the same setting as > the main application. Do you have any reason to have a library running with > CET, but the application does not have CET? Sure, using old binaries. That's why IBT has a legacy bitmap and things like MPX had ways of jumping into old non-enabled binaries. >> Can different threads in a process have different CET enabling state? > > Yes, if the parent starts with CET, children can turn it off. How would that work, though? clone() by default will copy the parent xsave state, which means it will be CET-enabled, which means it needs a shadow stack. So, if I want a CET-free child thread, I need to clone(), then turn CET off, then free the shadow stack? >> Does this *code* work? Could you please indicate which JITs have been >> enabled to use the code in this series? How much of the new ABI is in use? > > JIT does not necessarily use all of the ABI. The JIT changes mainly fix stack > frames and insert ENDBRs. I do not work on JIT. What I found is LLVM JIT fixes > are tested and in the master branch. Sljit fixes are in the release. Huh, so who is using the new prctl() ABIs? >> Where are the selftests/ for this new ABI? Were you planning on >> submitting any with this series? > > The ABI is more related to the application side, and therefore most suitable for > GLIBC unit tests. I was mostly concerned with the kernel selftests. The things in tools/testing/selftests/x86 in the kernel tree. > The more complicated areas such as pthreads, signals, ucontext, > fork() are all included there. I have been constantly running these > tests without any problems. I can provide more details if testing is > the concern. For something this complicated, with new kernel ABIs, we need an in-kernel sefltest. MPX was not that much different from this feature. It required a boatload of compiler and linker changes to function. Yet, there was a simple in-kernel test for it that didn't require *any* of that big pile of toolchain bits. Is there a reason we don't have one of those for CET?
