linux-kernel - Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack protection

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Date:   Fri, 06 Mar 2020 13:16:55 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...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>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for
 user-mode Shadow Stack protection

On Fri, 2020-03-06 at 11:02 -0800, Dave Hansen wrote:
> On 3/6/20 10:37 AM, Yu-cheng Yu wrote:
> > We used to do this for CET instructions, but after adding kernel-mode
> > instructions and inserting ENDBR's, the code becomes cluttered.  I also
> > found an earlier discussion on the ENDBR:
> > 
> > https://lore.kernel.org/lkml/CALCETrVRH8LeYoo7V1VBPqg4WS0Enxtizt=T7dPvgoeWfJrdzA@mail.gmail.com/
> > 
> > It makes sense to let the user know early on that the system cannot support
> > CET and cannot build a CET-enabled kernel.
> > 
> > One thing we can do is to disable CET in Kconfig and not in kernel
> > build, which I will do in the next version.
> 
> I'll go on the record and say I think we should allow building
> CET-enabled kernels on old toolchains.  We need it for build test
> coverage.  We can spit out a warning, but we need to allow building it.

The build test will go through (assembler or .byte), once the opcode patch
is applied [1].  Also, when we enable kernel-mode CET, it is difficult to
build IBT code without the right tool chain.

Yu-cheng

[1] opcode patch: 
https://lore.kernel.org/lkml/20200204171425.28073-1-yu-cheng.yu@intel.com/