On Thu, May 6, 2021 at 3:05 PM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote: > > On 5/4/2021 1:49 PM, Yu, Yu-cheng wrote: > > On 4/30/2021 11:32 AM, Yu, Yu-cheng wrote: > >> On 4/30/2021 10:47 AM, Andy Lutomirski 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. > >>>>> > > [...] > > >>>>> > >>>>> 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. > >>> > >>>> > >>>> However, the WAIT_ENDBR status needs to be saved/restored for signals. > >>>> Since IBT is now dependent on shadow stack, we can use a spare bit of > >>>> the shadow stack restore token for that. > >>> > >>> That seems like unnecessary ABI coupling. We have plenty of bits in > >>> uc_flags, and we have an entire reserved word in sigcontext. How > >>> about just sticking this bit in one of those places? > >> > >> Yes, I will make it UC_WAIT_ENDBR. > > > > Personally, I think an explicit flag is cleaner than using a reserved > > word somewhere. However, there is a small issue: ia32 has no uc_flags. > > > > This series can support legacy apps up to now. But, instead of creating > > too many special cases, perhaps we should drop CET support of ia32? > > > > Thoughts? I'm really not thrilled about coupling IBT and SHSTK like this. Here are a couple of possible solutions: - Don't support IBT in 32-bit mode, or maybe just don't support IBT with legacy 32-bit signals. The actual mechanics of this could be awkward. Maybe we would reject the sigaction() call or the IBT-enabling request if they conflict? - Find some space in the signal frame for these flags. Looking around a bit, sigframe_ia32 has fpstate_unused, but I can imagine things like CRIU getting very confused if it stops being unused. sigframe_ia32 uses sigcontext_32, which has a bunch of reserved space in __gsh, __fsh, etc. rt_sigframe_ia32 has uc_flags, so this isn't a real problem. I don't have a brilliant solution here.
