Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)

Yu, Yu-cheng
2021-05-03T20:25:53Z

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