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

[Andy Lutomirski <luto@kernel.org>]

On Tue, Sep 29, 2020 at 11:37 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> 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@intel.com> 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@intel.com> 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@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.
> +                        */

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.

--Andy