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

[Yu, Yu-cheng]

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.

Yu-cheng