Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation

[Andy Lutomirski]

On Tue, Oct 6, 2020 at 12:09 PM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On 10/1/2020 10:26 AM, Andy Lutomirski wrote:
> > On Thu, Oct 1, 2020 at 9:51 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
> >>
> >> On 9/30/2020 6:10 PM, Andy Lutomirski wrote:
> >>> On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> >>>>
> >>>> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >>
> >> [...]
> >>
> >>>>>>>>>     From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001
> >>>>>>>>> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> >>>>>>>>> Date: Thu, 29 Nov 2018 14:15:38 -0800
> >>>>>>>>> Subject: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and
> >>>>>>>>> Indirect Branch
> >>>>>>>>>      Tracking for vsyscall emulation
> >>>>>>>>>
> >>>>>>>>> Vsyscall entry points are effectively branch targets.  Mark them with
> >>>>>>>>> ENDBR64 opcodes.  When emulating the RET instruction, unwind shadow stack
> >>>>>>>>> and reset IBT state machine.
> >>>>>>>>>
> >>>>>>>>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
>
> [...]
>
> >>>>>>>>
> >>>>>>>
> >>>>>>> For what it's worth, I think there is an alternative.  If you all
> >>>>>>> (userspace people, etc) can come up with a credible way for a user
> >>>>>>> program to statically declare that it doesn't need vsyscalls, then we
> >>>>>>> could make SHSTK depend on *that*, and we could avoid this mess.  This
> >>>>>>> breaks orthogonality, but it's probably a decent outcome.
> >>>>>>>
> >>>>>>
> >>>>>> Would an arch_prctl(DISABLE_VSYSCALL) work?  The kernel then sets a
> >>>>>> thread flag, and in emulate_vsyscall(), checks the flag.
> >>>>>>
> >>>>>> When CET is enabled, ld-linux will do DISABLE_VSYSCALL.
> >>>>>>
> >>>>>> How is that?
> >>>>>
> >>>>> Backwards, no?  Presumably vsyscall needs to be disabled before or
> >>>>> concurrently with CET being enabled, not after.
> >>>>>
> >>>>> I think the solution of making vsyscall emulation work correctly with
> >>>>> CET is going to be better and possibly more straightforward.
> >>>>>
> >>>>
> >>>> We can do
> >>>>
> >>>> 1. Add ARCH_X86_DISABLE_VSYSCALL to disable the vsyscall page.
> >>>> 2. If CPU supports CET and the program is CET enabled:
> >>>>       a. Disable the vsyscall page.
> >>>>       b. Pass control to user.
> >>>>       c. Enable the vsyscall page when ARCH_X86_CET_DISABLE is called.
> >>>>
> >>>> So when control is passed from kernel to user, the vsyscall page is
> >>>> disabled if the program
> >>>> is CET enabled.
> >>>
> >>> Let me say this one more time:
> >>>
> >>> If we have a per-process vsyscall disable control and a per-process
> >>> CET control, we are going to keep those settings orthogonal.  I'm
> >>> willing to entertain an option in which enabling SHSTK without also
> >>> disabling vsyscalls is disallowed, We are *not* going to have any CET
> >>> flags magically disable vsyscalls, though, and we are not going to
> >>> have a situation where disabling vsyscalls on process startup requires
> >>> enabling SHSTK.
> >>>
> >>> Any possible static vsyscall controls (and CET controls, for that
> >>> matter) also need to come with some explanation of whether they are
> >>> properties set on the ELF loader, the ELF program being loaded, or
> >>> both.  And this explanation needs to cover what happens when old
> >>> binaries link against new libc versions and vice versa.  A new
> >>> CET-enabled binary linked against old libc running on a new kernel
> >>> that is expected to work on a non-CET CPU MUST work on a CET CPU, too.
> >>>
> >>> Right now, literally the only thing preventing vsyscall emulation from
> >>> coexisting with SHSTK is that the implementation eeds work.
> >>>
> >>> So your proposal is rejected.  Sorry.
> >>>
> >> I think, even with shadow stack/ibt enabled, we can still allow XONLY
> >> without too much mess.
> >>
> >> What about this?
> >>
> >> Thanks,
> >> Yu-cheng
> >>
> >> ======
> >>
> >> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
> >> b/arch/x86/entry/vsyscall/vsyscall_64.c
> >> index 8b0b32ac7791..d39da0a15521 100644
> >> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> >> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> >> @@ -48,16 +48,16 @@
> >>    static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
> >>    #ifdef CONFIG_LEGACY_VSYSCALL_NONE
> >>           NONE;
> >> -#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
> >> +#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY) || defined(CONFIG_X86_CET)
> >>           XONLY;
> >> -#else
> >> +#else
> >>           EMULATE;
> >>    #endif
> >
> > I don't get it.
> >
> > First, you can't do any of this based on config -- it must be runtime.
> >
> > Second, and more importantly, I don't see how XONLY helps at all.  The
> > (non-executable) text that's exposed to user code in EMULATE mode is
> > trivial to get right with CET -- your code already handles it.  It's
> > the emulation code (that runs identically in EMULATE and XONLY mode)
> > that's tricky.
> >
>
> Hi,
>
> There has been some ambiguity in my previous proposals.  To make things
> clear, I created a patch for arch_prctl(VSYSCALL_CTL), which controls
> the TIF_VSYSCALL_DISABLE flag.  It is entirely orthogonal to shadow
> stack or IBT.  On top of the patch, we can do SET_PERSONALITY2() to
> disable vsyscall, e.g.

NAK.  Let me try explaining again.

>
> ======
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 0e1be2a13359..c730ff00bc62 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -394,6 +394,19 @@ struct arch_elf_state {
>         .gnu_property = 0,      \
>   }
>
> +#define SET_PERSONALITY2(ex, state)                            \
> +do {                                                           \
> +       unsigned int has_cet;                                   \
> +                                                               \
> +       has_cet = GNU_PROPERTY_X86_FEATURE_1_SHSTK |            \
> +                 GNU_PROPERTY_X86_FEATURE_1_IBT;               \
> +                                                               \
> +       if ((state)->gnu_property & has_cet)                    \
> +               set_thread_flag(TIF_VSYSCALL_DISABLE);          \
> +                                                               \
> +       SET_PERSONALITY(ex);                                    \
> +} while (0)
> +

This is not what "orthogonal" means.  If the bits were orthogonal, the
logic would be:

if (gnu_property & DISABLE_VSYSCALL)
  disable vsyscall;
if (gnu_property & SHSTK)
  enable SHSTK;
if (gnu_property & IBT);
  enable IBT;

and, if necessarily (although I still think it would be preferable not
to do this):

if ((gnu_property & (DISABLE_VSYSCALL | SHSTK)) == SHSTK)
  return -EINVAL;

As far as I'm concerned, you have two choices:

a) Make SHSTK work *correctly* with vsyscall emulation.

b) Add a high quality mechanism to disable vsyscall emulation and make
SHSTK depend on that.

As far as I'm concerned, (a) is preferable.  Ideally we'd get (a)
*and* a high quality vsyscall emulation disable mechanism with no
dependencies.


> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
> b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 44c33103a955..fe8f3db6d21b 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -127,6 +127,9 @@ bool emulate_vsyscall(unsigned long error_code,
>         long ret;
>         unsigned long orig_dx;
>
> +       if (test_thread_flag(TIF_VSYSCALL_DISABLE))
> +               return false;
> +

This needs to be per-mm, not per-thread.  There's a patch floating
around that gets us about a quarter of the way there.  I'm not
convinced that CET should wait for this to finish.