linux-kernel - Re: [PATCH v10 01/26] Documentation/x86: Add CET description

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Date:   Mon, 18 May 2020 16:47:42 -0700
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>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v10 01/26] Documentation/x86: Add CET description

On Fri, 2020-05-15 at 19:53 -0700, Yu-cheng Yu wrote:
> On Fri, 2020-05-15 at 16:56 -0700, Dave Hansen wrote:
> > On 5/15/20 4:29 PM, Yu-cheng Yu wrote:
> > > [...]
> > > I have run them with CET enabled.  All of them pass, except for the following:
> > > Sigreturn from 64-bit to 32-bit fails, because shadow stack is at a 64-bit
> > > address.  This is understandable.
> > [...]
> > One a separate topic: You ran the selftests and one failed.  This is a
> > *MASSIVE* warning sign.  It should minimally be described in your cover
> > letter, and accompanied by a fix to the test case.  It is absolutely
> > unacceptable to introduce a kernel feature that causes a test to fail.
> > You must either fix your kernel feature or you fix the test.
> > 
> > This code can not be accepted until this selftests issue is rectified.

The x86/sigreturn test constructs 32-bit ldt entries, and does sigreturn from
64-bit to 32-bit context.  We do not have a way to construct a static 32-bit
shadow stack.  Why do we want that?  I think we can simply run the test with CET
disabled.

Yu-cheng