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SubjectRe: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation
On Tue, Sep 29, 2020 at 12:57 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> 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.
>

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.

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