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

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Message-ID: <20180830203948.GB1936@amd>
Date:   Thu, 30 Aug 2018 22:39:48 +0200
From:   Pavel Machek <pavel@....cz>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
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

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.
 
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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