linux-kernel - Re: [RFC PATCH v3 05/24] Documentation/x86: Add CET description

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Date:   Thu, 30 Aug 2018 15:49:51 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Pavel Machek <pavel@....cz>
Cc:     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@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.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@...omiun.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>
Subject: Re: [RFC PATCH v3 05/24] Documentation/x86: Add CET description

On Thu, 2018-08-30 at 22:39 +0200, Pavel Machek wrote:
> Hi!
> 
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt
> > b/Documentation/admin-guide/kernel-parameters.txt
> > index 9871e649ffef..b090787188b4 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2764,6 +2764,12 @@
> >  			noexec=on: enable non-executable mappings
> > (default)
> >  			noexec=off: disable non-executable
> > mappings
> >  
> > +	no_cet_ibt	[X86-64] Disable indirect branch
> > tracking for user-mode
> > +			applications
> > +
> > +	no_cet_shstk	[X86-64] Disable shadow stack support
> > for user-mode
> > +			applications
> Hmm, not too consistent with "nosmap" below. Would it make sense to
> have cet=on/off/ibt/shstk instead?
> 
> > 
> > +++ b/Documentation/x86/intel_cet.rst
> > @@ -0,0 +1,252 @@
> > +=========================================
> > +Control Flow Enforcement Technology (CET)
> > +=========================================
> > +
> > +[1] Overview
> > +============
> > +
> > +Control Flow Enforcement Technology (CET) provides protection
> > against
> > +return/jump-oriented programing (ROP) attacks.
> Can you add something like "It attempts to protect process from
> running arbitrary code even after attacker has control of its stack"
> -- for people that don't know what ROP is, and perhaps link to
> wikipedia explaining ROP or something...
> 
> > 
> > It can be implemented
> > +to protect both the kernel and applications.  In the first phase,
> > +only the user-mode protection is implemented for the 64-bit
> > kernel.
> > +Thirty-two bit applications are supported under the compatibility
> 32-bit (for consistency).
> 
> Ok, so CET stops execution of malicious code before architectural
> effects are visible, correct? Does it prevent micro-architectural
> effects of the malicious code? (cache content would be one example;
> see Spectre).
> 
> > 
> > +[3] Application Enabling
> > +========================
> "Enabling CET in applications" ?
> 
> > 
> > +Signal
> > +------
> > +
> > +The main program and its signal handlers use the same
> > SHSTK.  Because
> > +the SHSTK stores only return addresses, we can estimate a large
> > +enough SHSTK to cover the condition that both the program stack
> > and
> > +the sigaltstack run out.
> English? Is it estimate or is it large enough? "a large" -- "a"
> should
> be deleted AFAICT.
>  

I will work on these, thanks!