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

From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Pavel Machek <pavel@ucw.cz>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, linux-arch@vger.kernel.org,
	linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
	Andy Lutomirski <luto@amacapital.net>,
	Balbir Singh <bsingharora@gmail.com>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromiun.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Subject: Re: [RFC PATCH v3 05/24] Documentation/x86: Add CET description
Date: Thu, 30 Aug 2018 15:49:51 -0700	[thread overview]
Message-ID: <1535669391.28781.7.camel@intel.com> (raw)
In-Reply-To: <20180830203948.GB1936@amd>

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!