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

[H.J. Lu]

On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Wed, Sep 30, 2020 at 3:33 PM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
> >
> > On 9/29/2020 1:00 PM, Andy Lutomirski wrote:
> > > On Tue, Sep 29, 2020 at 12:57 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > >>
> > >> On Tue, Sep 29, 2020 at 11:37 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
> > >>>
> > >>> On 9/28/2020 10:37 AM, Andy Lutomirski wrote:
> > >>>> On Mon, Sep 28, 2020 at 9:59 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > >>>>>
> > >>>>> On Fri, 2020-09-25 at 09:51 -0700, Andy Lutomirski wrote:
> > >>>>>>> On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
> > >>>>> +
> > >>>>> +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> > >>>>> +               if (!cet) {
> > >>>>> +                       /*
> > >>>>> +                        * This is an unlikely case where the task is
> > >>>>> +                        * CET-enabled, but CET xstate is in INIT.
> > >>>>> +                        */
> > >>>>> +                       WARN_ONCE(1, "CET is enabled, but no xstates");
> > >>>>
> > >>>> "unlikely" doesn't really cover this.
> > >>>>
> > >>>>> +                       fpregs_unlock();
> > >>>>> +                       goto sigsegv;
> > >>>>> +               }
> > >>>>> +
> > >>>>> +               if (cet->user_ssp && ((cet->user_ssp + 8) < TASK_SIZE_MAX))
> > >>>>> +                       cet->user_ssp += 8;
> > >>>>
> > >>>> This looks buggy.  The condition should be "if SHSTK is on, then add 8
> > >>>> to user_ssp".  If the result is noncanonical, then some appropriate
> > >>>> exception should be generated, probably by the FPU restore code -- see
> > >>>> below.  You should be checking the SHSTK_EN bit, not SSP.
> > >>>
> > >>> Updated.  Is this OK?  I will resend the whole series later.
> > >>>
> > >>> Thanks,
> > >>> Yu-cheng
> > >>>
> > >>> ======
> > >>>
> > >>>   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>
> > >>> ---
> > >>> 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.
> > >>> +                        */
> > >>
> > >> Can the comment explain better, please?  I would say something like:
> > >>
> > >> If the kernel thinks this task has CET enabled (because
> > >> tsk->thread.cet has one of the features enabled), then the
> > >> corresponding bits must also be set in the CET XSAVES region.  If the
> > >> CET XSAVES region is in the INIT state, then the kernel's concept of
> > >> the task's CET state is corrupt.
> > >>
> > >>> +                       WARN_ONCE(1, "CET is enabled, but no xstates");
> > >>> +                       fpregs_unlock();
> > >>> +                       goto sigsegv;
> > >>> +               }
> > >>> +
> > >>> +               if (cet->user_cet & CET_SHSTK_EN) {
> > >>> +                       if (cet->user_ssp && (cet->user_ssp + 8 < TASK_SIZE_MAX))
> > >>> +                               cet->user_ssp += 8;
> > >>> +               }
> > >>
> > >> This makes so sense to me.  Also, the vsyscall emulation code is
> > >> intended to be as rigid as possible to minimize the chance that it
> > >> gets used as an exploit gadget.  So we should not silently corrupt
> > >> anything.  Moreover, this code seems quite dangerous -- you've created
> > >> a gadget that does RET without actually verifying the SHSTK token.  If
> > >> SHSTK and some form of strong indirect branch/call CFI is in use, then
> > >> the existance of a CFI-bypassing return primitive at a fixed address
> > >> seems quite problematic.
> > >>
> > >> So I think you need to write a function that reasonably accurately
> > >> emulates a usermode RET.
> > >>
> > >
> > > 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.

-- 
H.J.