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

[Andy Lutomirski]

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:
> >
> > The kernel has:
> >
> > struct rt_sigframe {
> >      char __user *pretcode;
> >      struct ucontext uc;
> >      struct siginfo info;
> >      /* fp state follows here */
> > };
> >
> > This is roughly the actual signal frame.  But userspace does not have
> > this struct declared, and user code does not know the sizes of the
> > fields.  So it's accessed in a nonsensical way.  The signal handler
> > function is passed a pointer to the whole sigframe implicitly in RSP,
> > a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX.
> > User code can *find* the fp state by following a pointer from
> > mcontext, which is, in turn, found via uc:
> >
> > struct ucontext {
> >      unsigned long      uc_flags;
> >      struct ucontext  *uc_link;
> >      stack_t          uc_stack;
> >      struct sigcontext uc_mcontext;  <-- fp pointer is in here
> >      sigset_t      uc_sigmask;    /* mask last for extensibility */
> > };
> >
> > The kernel, in sigreturn, works a bit differently.  The sigreturn
> > variants know the base address of the frame but don't have the benefit
> > of receiving pointers to the fields.  So instead the kernel takes
> > advantage of the fact that it knows the offset to uc and parses uc
> > accordingly.  And the kernel follows the pointer in mcontext to find
> > the fp state.  The latter bit is quite important later.  The kernel
> > does not parse info at all.
> >
> > The fp state is its own mess.  When XSAVE happened, Intel kindly (?)
> > gave us a software defined area between the "legacy" x87 region and
> > the modern supposedly extensible part.  Linux sticks the following
> > structure in that hole:
> >
> > struct _fpx_sw_bytes {
> >      /*
> >       * If set to FP_XSTATE_MAGIC1 then this is an xstate context.
> >       * 0 if a legacy frame.
> >       */
> >      __u32                magic1;
> >
> >      /*
> >       * Total size of the fpstate area:
> >       *
> >       *  - if magic1 == 0 then it's sizeof(struct _fpstate)
> >       *  - if magic1 == FP_XSTATE_MAGIC1 then it's sizeof(struct _xstate)
> >       *    plus extensions (if any)
> >       */
> >      __u32                extended_size;
> >
> >      /*
> >       * Feature bit mask (including FP/SSE/extended state) that is present
> >       * in the memory layout:
> >       */
> >      __u64                xfeatures;
> >
> >      /*
> >       * Actual XSAVE state size, based on the xfeatures saved in the layout.
> >       * 'extended_size' is greater than 'xstate_size':
> >       */
> >      __u32                xstate_size;
> >
> >      /* For future use: */
> >      __u32                padding[7];
> > };
> >
> >
> > 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?

--Andy