Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

[Andy Lutomirski]

On Mon, Sep 21, 2020 at 3:37 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On Mon, 2020-09-21 at 09:22 -0700, Yu, Yu-cheng wrote:
> > On 9/18/2020 5:11 PM, Andy Lutomirski wrote:
> > > On Fri, Sep 18, 2020 at 12:23 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > > > Emulation of the legacy vsyscall page is required by some programs
> > > > built before 2013.  Newer programs after 2013 don't use it.
> > > > Disable vsyscall emulation when Control-flow Enforcement (CET) is
> > > > enabled to enhance security.
> > > >
> > > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> [...]
> > >
> > > Nope, try again.  Having IBT on does *not* mean that every library in
> > > the process knows that we have indirect branch tracking.  The legacy
> > > bitmap exists for a reason.  Also, I want a way to flag programs as
> > > not using the vsyscall page, but that flag should not be called CET.
> > > And a process with vsyscalls off should not be able to read the
> > > vsyscall page, and /proc/self/maps should be correct.
> > >
> > > So you have some choices:
> > >
> > > 1. Drop this patch and make it work.
> > >
> > > 2. Add a real per-process vsyscall control.  Either make it depend on
> > > vsyscall=xonly and wire it up correctly or actually make it work
> > > correctly with vsyscall=emulate.
> > >
> > > NAK to any hacks in this space.  Do it right or don't do it at all.
> > >
> >
> > We can drop this patch, and bring back the previous patch that fixes up
> > shadow stack and ibt.  That makes vsyscall emulation work correctly, and
> > does not force the application to do anything different from what is
> > working now.  I will post the previous patch as a reply to this thread
> > so that people can make comments on it.
> >
> > Yu-cheng
>
> Here is the patch:
>
> ------
>
> From dfdee39c795ee5dcee2c77f6ba344a61f4d8124b 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 34/43] 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 the shadow
> stack and reset IBT state machine.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/entry/vsyscall/vsyscall_64.c     | 29 +++++++++++++++++++++++
>  arch/x86/entry/vsyscall/vsyscall_emu_64.S |  9 +++++++
>  arch/x86/entry/vsyscall/vsyscall_trace.h  |  1 +
>  3 files changed, 39 insertions(+)
>
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
> b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 44c33103a955..0131c9f7f9c5 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 +289,32 @@ bool emulate_vsyscall(unsigned long error_code,
>         /* Emulate a ret instruction. */
>         regs->ip = caller;
>         regs->sp += 8;
> +
> +       if (current->thread.cet.shstk_size ||
> +           current->thread.cet.ibt_enabled) {
> +               u64 r;
> +
> +               fpregs_lock();
> +               if (test_thread_flag(TIF_NEED_FPU_LOAD))
> +                       __fpregs_load_activate();

Wouldn't this be nicer if you operated on the memory image, not the registers?

> +
> +#ifdef CONFIG_X86_INTEL_BRANCH_TRACKING_USER
> +               /* Fixup branch tracking */
> +               if (current->thread.cet.ibt_enabled) {
> +                       rdmsrl(MSR_IA32_U_CET, r);
> +                       wrmsrl(MSR_IA32_U_CET, r & ~CET_WAIT_ENDBR);
> +               }
> +#endif

Seems reasonable on first glance.

> +
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> +               /* Unwind shadow stack. */
> +               if (current->thread.cet.shstk_size) {
> +                       rdmsrl(MSR_IA32_PL3_SSP, r);
> +                       wrmsrl(MSR_IA32_PL3_SSP, r + 8);
> +               }
> +#endif

What happens if the result is noncanonical?  A quick skim of the SDM
didn't find anything.  This latter issue goes away if you operate on
the memory image, though -- writing a bogus value is just fine, since
the FP restore will handle it.