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Subject: Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES
 supervisor states
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>, 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@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.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@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>
References: <20210127212524.10188-1-yu-cheng.yu@intel.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <0a5a80c0-afc7-5f91-9e28-a300e30f1ab3@intel.com>
Date:   Fri, 29 Jan 2021 14:53:12 -0800
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Precedence: bulk

On 1/29/21 2:35 PM, Yu, Yu-cheng wrote:
>> Andy Cooper just mentioned on IRC about this nugget in the spec:
>>
>>     XRSTORS on CET state will do reserved bit and canonicality
>>     checks on the state in similar manner as done by the WRMSR to
>>     these state elements.
>>
>> We're using copy_kernel_to_xregs_err(), so the #GP *should* be OK.
>> Could we prove this out in practice, please?
>>>
> Do we want to verify that setting reserved bits in CET XSAVES states
> triggers GP?  Then, yes, I just verified it again.  Thanks for
> reminding.  Do we have any particular case relating to this?

I want to confirm that it triggers #GP and kills userspace without the
kernel WARN'ing or otherwise being visibly unhappy.

What about the return-to-userspace path after a ptracer writes content
to the CET fields?   I don't see the same tolerance for errors in
__fpregs_load_activate(), for instance.