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

From: Andy Lutomirski
Date: Wed Sep 30 2020 - 19:44:55 EST

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.

>
> Yu-cheng