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

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Message-ID: <20200918205933.GB4304@duo.ucw.cz>
Date:   Fri, 18 Sep 2020 22:59:33 +0200
From:   Pavel Machek <pavel@....cz>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v12 1/8] x86/cet/ibt: Add Kconfig option for user-mode
 Indirect Branch Tracking

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@...el.com>
> > ---
> > 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, ...." :-) ?

Best regards,
									Pavel


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