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

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

On 9/30/2020 6:10 PM, Andy Lutomirski wrote:
> On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <luto@kernel.org> wrote:

[...]

>>>>>>>    From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001
>>>>>>> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>>>>>>> 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@intel.com>
>>>>>>> ---
>>>>>>> v13:
>>>>>>> - Check shadow stack address is canonical.
>>>>>>> - Change from writing to MSRs to writing to CET xstate.
>>>>>>>
>>>>>>>     arch/x86/entry/vsyscall/vsyscall_64.c     | 34 +++++++++++++++++++++++
>>>>>>>     arch/x86/entry/vsyscall/vsyscall_emu_64.S |  9 ++++++
>>>>>>>     arch/x86/entry/vsyscall/vsyscall_trace.h  |  1 +
>>>>>>>     3 files changed, 44 insertions(+)
>>>>>>>
>>>>>>> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
>>>>>>> b/arch/x86/entry/vsyscall/vsyscall_64.c
>>>>>>> index 44c33103a955..30b166091d46 100644
>>>>>>> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
>>>>>>> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
>>>>>>> @@ -38,6 +38,9 @@
>>>>>>>     #include <asm/fixmap.h>
>>>>>>>     #include <asm/traps.h>
>>>>>>>     #include <asm/paravirt.h>
>>>>>>> +#include <asm/fpu/xstate.h>
>>>>>>> +#include <asm/fpu/types.h>
>>>>>>> +#include <asm/fpu/internal.h>
>>>>>>>
>>>>>>>     #define CREATE_TRACE_POINTS
>>>>>>>     #include "vsyscall_trace.h"
>>>>>>> @@ -286,6 +289,44 @@ bool emulate_vsyscall(unsigned long error_code,
>>>>>>>           /* Emulate a ret instruction. */
>>>>>>>           regs->ip = caller;
>>>>>>>           regs->sp += 8;
>>>>>>> +
>>>>>>> +#ifdef CONFIG_X86_CET
>>>>>>> +       if (tsk->thread.cet.shstk_size || tsk->thread.cet.ibt_enabled) {
>>>>>>> +               struct cet_user_state *cet;
>>>>>>> +               struct fpu *fpu;
>>>>>>> +
>>>>>>> +               fpu = &tsk->thread.fpu;
>>>>>>> +               fpregs_lock();
>>>>>>> +
>>>>>>> +               if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
>>>>>>> +                       copy_fpregs_to_fpstate(fpu);
>>>>>>> +                       set_thread_flag(TIF_NEED_FPU_LOAD);
>>>>>>> +               }
>>>>>>> +
>>>>>>> +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>>>>>>> +               if (!cet) {
>>>>>>> +                       /*
>>>>>>> +                        * This should not happen.  The task is
>>>>>>> +                        * CET-enabled, but CET xstate is in INIT.
>>>>>>> +                        */
>>>>>>
[...]
>>>>>>
>>>>>
>>>>> 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

  static int __init vsyscall_setup(char *str)
  {
         if (str) {
-               if (!strcmp("emulate", str))
+               if (!strcmp("emulate", str) && !IS_ENABLED(CONFIG_X86_CET))
                         vsyscall_mode = EMULATE;
                 else if (!strcmp("xonly", str))
                         vsyscall_mode = XONLY;