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

[Andy Lutomirski]

> 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