Messages in this thread | | | Subject | Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack) | From | "Yu, Yu-cheng" <> | Date | Mon, 3 May 2021 13:25:45 -0700 |
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On 5/3/2021 8:29 AM, Andy Lutomirski wrote: > >> On May 3, 2021, at 8:14 AM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote: >> >> On 5/2/2021 4:23 PM, Andy Lutomirski wrote: >>>> On Fri, Apr 30, 2021 at 10:47 AM Andy Lutomirski <luto@kernel.org> wrote: >>>> >>>> On Fri, Apr 30, 2021 at 10:00 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote: >>>>> >>>>> On 4/28/2021 4:03 PM, Andy Lutomirski wrote: >>>>>> On Tue, Apr 27, 2021 at 1:44 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote: >>>>>>> >>>>>>> When shadow stack is enabled, a task's shadow stack states must be saved >>>>>>> along with the signal context and later restored in sigreturn. However, >>>>>>> currently there is no systematic facility for extending a signal context. >>>>>>> There is some space left in the ucontext, but changing ucontext is likely >>>>>>> to create compatibility issues and there is not enough space for further >>>>>>> extensions. >>>>>>> >>>>>>> Introduce a signal context extension struct 'sc_ext', which is used to save >>>>>>> shadow stack restore token address. The extension is located above the fpu >>>>>>> states, plus alignment. The struct can be extended (such as the ibt's >>>>>>> wait_endbr status to be introduced later), and sc_ext.total_size field >>>>>>> keeps track of total size. >>>>>> >>>>>> I still don't like this. >>>>>> >>>>>> Here's how the signal layout works, for better or for worse: >>>>>> >> >> [...] >> >>>>>> >>>>>> That's where we are right now upstream. The kernel has a parser for >>>>>> the FPU state that is bugs piled upon bugs and is going to have to be >>>>>> rewritten sometime soon. On top of all this, we have two upcoming >>>>>> features, both of which require different kinds of extensions: >>>>>> >>>>>> 1. AVX-512. (Yeah, you thought this story was over a few years ago, >>>>>> but no. And AMX makes it worse.) To make a long story short, we >>>>>> promised user code many years ago that a signal frame fit in 2048 >>>>>> bytes with some room to spare. With AVX-512 this is false. With AMX >>>>>> it's so wrong it's not even funny. The only way out of the mess >>>>>> anyone has come up with involves making the length of the FPU state >>>>>> vary depending on which features are INIT, i.e. making it more compact >>>>>> than "compact" mode is. This has a side effect: it's no longer >>>>>> possible to modify the state in place, because enabling a feature with >>>>>> no space allocated will make the structure bigger, and the stack won't >>>>>> have room. Fortunately, one can relocate the entire FPU state, update >>>>>> the pointer in mcontext, and the kernel will happily follow the >>>>>> pointer. So new code on a new kernel using a super-compact state >>>>>> could expand the state by allocating new memory (on the heap? very >>>>>> awkwardly on the stack?) and changing the pointer. For all we know, >>>>>> some code already fiddles with the pointer. This is great, except >>>>>> that your patch sticks more data at the end of the FPU block that no >>>>>> one is expecting, and your sigreturn code follows that pointer, and >>>>>> will read off into lala land. >>>>>> >>>>> >>>>> Then, what about we don't do that at all. Is it possible from now on we >>>>> don't stick more data at the end, and take the relocating-fpu approach? >>>>> >>>>>> 2. CET. CET wants us to find a few more bytes somewhere, and those >>>>>> bytes logically belong in ucontext, and here we are. >>>>>> >>>>> >>>>> Fortunately, we can spare CET the need of ucontext extension. When the >>>>> kernel handles sigreturn, the user-mode shadow stack pointer is right at >>>>> the restore token. There is no need to put that in ucontext. >>>> >>>> That seems entirely reasonable. This might also avoid needing to >>>> teach CRIU about CET at all. >>> Wait, what's the actual shadow stack token format? And is the token >>> on the new stack or the old stack when sigaltstack is in use? For >>> that matter, is there any support for an alternate shadow stack for >>> signals? >> >> The restore token is a pointer pointing directly above itself and bit[0] indicates 64-bit mode. >> >> Because the shadow stack stores only return addresses, there is no alternate shadow stack. However, the application can allocate and switch to a new shadow stack. > > I think we should make the ABI support an alternate shadow stack even if we don’t implement it initially. After all, some day someone might want to register a handler for shadow stack overflow. >
Agree. We can probably add something in parallel of sigaltstack(), and let the user choose separately alternate normal/shadow stacks.
Thanks, Yu-cheng
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