> On May 3, 2021, at 8:14 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > > On 5/2/2021 4:23 PM, Andy Lutomirski wrote: >>> On Fri, Apr 30, 2021 at 10:47 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>> >>> On Fri, Apr 30, 2021 at 10:00 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> 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@xxxxxxxxx> 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. > > Yu-cheng
