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 08:13:57 -0700 |
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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: >>>>
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>>>> >>>> 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.
Yu-cheng
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