linux-kernel - Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET

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Message-ID: <3967f126-f7ea-36fd-bec0-dfbbc46ef221@intel.com>
Date:   Thu, 3 Sep 2020 09:21:10 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Jann Horn <jannh@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <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>, 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 v11 6/9] x86/cet: Add PTRACE interface for CET

On 9/3/2020 9:11 AM, Dave Hansen wrote:
> On 9/3/20 9:09 AM, Yu, Yu-cheng wrote:
>> If the debugger is going to write an MSR, only in the third case would
>> this make a slight sense.  For example, if the system has CET enabled,
>> but the task does not have CET enabled, and GDB is writing to a CET MSR.
>>   But still, this is strange to me.
> 
> If this is strange, then why do we even _implement_ writes?
> 

For example, if the task has CET enabled, and it is in a control 
protection fault, the debugger can clear the task's IBT state, or unwind 
the shadow stack, etc.  But if the task does not have CET enabled (its 
CET MSRs are in INIT state), it would make sense for the PTRACE call to 
return failure than doing something else, right?