Re: [PATCH v12 1/8] x86/cet/ibt: Add Kconfig option for user-mode Indirect Branch Tracking

[H.J. Lu]

On Fri, Sep 18, 2020 at 2:40 PM Pavel Machek <pavel@xxxxxx> wrote:
>
> On Fri 2020-09-18 14:25:12, Yu, Yu-cheng wrote:
> > On 9/18/2020 1:59 PM, Pavel Machek wrote:
> > > On Fri 2020-09-18 13:24:13, Randy Dunlap wrote:
> > > > Hi,
> > > >
> > > > If you do another version of this:
> > > >
> > > > On 9/18/20 12:23 PM, Yu-cheng Yu wrote:
> > > > > Introduce Kconfig option X86_INTEL_BRANCH_TRACKING_USER.
> > > > >
> > > > > Indirect Branch Tracking (IBT) provides protection against CALL-/JMP-
> > > > > oriented programming attacks.  It is active when the kernel has this
> > > > > feature enabled, and the processor and the application support it.
> > > > > When this feature is enabled, legacy non-IBT applications continue to
> > > > > work, but without IBT protection.
> > > > >
> > > > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > > > > ---
> > > > > v10:
> > > > > - Change build-time CET check to config depends on.
> > > > >
> > > > >   arch/x86/Kconfig | 16 ++++++++++++++++
> > > > >   1 file changed, 16 insertions(+)
> > > > >
> > > > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > > > > index 6b6dad011763..b047e0a8d1c2 100644
> > > > > --- a/arch/x86/Kconfig
> > > > > +++ b/arch/x86/Kconfig
> > > > > @@ -1963,6 +1963,22 @@ config X86_INTEL_SHADOW_STACK_USER
> > > > >           If unsure, say y.
> > > > > +config X86_INTEL_BRANCH_TRACKING_USER
> > > > > +       prompt "Intel Indirect Branch Tracking for user-mode"
> > > > > +       def_bool n
> > > > > +       depends on CPU_SUP_INTEL && X86_64
> > > > > +       depends on $(cc-option,-fcf-protection)
> > > > > +       select X86_INTEL_CET
> > > > > +       help
> > > > > +         Indirect Branch Tracking (IBT) provides protection against
> > > > > +         CALL-/JMP-oriented programming attacks.  It is active when
> > > > > +         the kernel has this feature enabled, and the processor and
> > > > > +         the application support it.  When this feature is enabled,
> > > > > +         legacy non-IBT applications continue to work, but without
> > > > > +         IBT protection.
> > > > > +
> > > > > +         If unsure, say y
> > > >
> > > >     If unsure, say y.
> > >
> > > Actually, it would be "If unsure, say Y.", to be consistent with the
> > > rest of the Kconfig.
> > >
> > > But I wonder if Yes by default is good idea. Only very new CPUs will
> > > support this, right? Are they even available at the market? Should the
> > > help text say "if your CPU is Whatever Lake or newer, ...." :-) ?
> >
> > I will revise the wording if there is another version.  But a CET-capable
> > kernel can run on legacy systems.  We have been testing that combination.
>
> Yes, but enabling CET is unneccessary overhead on older systems. And
> Kconfig is great place to explain that.
>

I can't tell any visible CET kernel overhead on my non-CET machines.

-- 
H.J.