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

[Yu, Yu-cheng]

On 5/3/2021 8:29 AM, Andy Lutomirski wrote:
> > 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.

Agree.  We can probably add something in parallel of sigaltstack(), and 
let the user choose separately alternate normal/shadow stacks.

Thanks,
Yu-cheng