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

[Andy Lutomirski <luto@kernel.org>]

On Thu, May 6, 2021 at 3:05 PM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>
> On 5/4/2021 1:49 PM, Yu, Yu-cheng wrote:
> > On 4/30/2021 11:32 AM, Yu, Yu-cheng wrote:
> >> On 4/30/2021 10:47 AM, Andy Lutomirski 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.
> >>>>>
>
> [...]
>
> >>>>>
> >>>>> 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.
> >>>
> >>>>
> >>>> However, the WAIT_ENDBR status needs to be saved/restored for signals.
> >>>> Since IBT is now dependent on shadow stack, we can use a spare bit of
> >>>> the shadow stack restore token for that.
> >>>
> >>> That seems like unnecessary ABI coupling.  We have plenty of bits in
> >>> uc_flags, and we have an entire reserved word in sigcontext.  How
> >>> about just sticking this bit in one of those places?
> >>
> >> Yes, I will make it UC_WAIT_ENDBR.
> >
> > Personally, I think an explicit flag is cleaner than using a reserved
> > word somewhere.  However, there is a small issue: ia32 has no uc_flags.
> >
> > This series can support legacy apps up to now.  But, instead of creating
> > too many special cases, perhaps we should drop CET support of ia32?
> >
> > Thoughts?

I'm really not thrilled about coupling IBT and SHSTK like this.

Here are a couple of possible solutions:

- Don't support IBT in 32-bit mode, or maybe just don't support IBT
with legacy 32-bit signals.  The actual mechanics of this could be
awkward.  Maybe we would reject the sigaction() call or the
IBT-enabling request if they conflict?

- Find some space in the signal frame for these flags.  Looking around
a bit, sigframe_ia32 has fpstate_unused, but I can imagine things like
CRIU getting very confused if it stops being unused.  sigframe_ia32
uses sigcontext_32, which has a bunch of reserved space in __gsh,
__fsh, etc.

rt_sigframe_ia32 has uc_flags, so this isn't a real problem.

I don't have a brilliant solution here.