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SubjectRe: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)
On 5/2/2021 4:23 PM, Andy Lutomirski wrote:
> On Fri, Apr 30, 2021 at 10:47 AM Andy Lutomirski <> wrote:
>> On Fri, Apr 30, 2021 at 10:00 AM Yu, Yu-cheng <> wrote:
>>> On 4/28/2021 4:03 PM, Andy Lutomirski wrote:
>>>> On Tue, Apr 27, 2021 at 1:44 PM Yu-cheng Yu <> 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.


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