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

[Andy Lutomirski]

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.