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

Andy Lutomirski
2021-05-03T15:29:16Z

> 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.

>
> Yu-cheng