linux-kernel - Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states

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Date:   Mon, 1 Feb 2021 15:05:16 -0800
From:   "Yu, Yu-cheng" <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>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES
 supervisor states

On 2/1/2021 2:59 PM, Dave Hansen wrote:
> On 2/1/21 2:43 PM, Yu, Yu-cheng wrote:
>> On 1/29/2021 2:53 PM, Dave Hansen wrote:
>>> 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.
>>
>> For sigreturn, shadow stack pointer is checked against its restore token
>> and must be smaller than TASK_SIZE_MAX.  Sigreturn cannot set any
>> MSR_IA32_U_CET reserved bits.
> 
> That would be nice to at least allude to in the changelog or comments.
> 

Ok.

>>> 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.
>>>
>>
>> Good thought.  I have not sent out my revised PTRACE patch, but values
>> from user will be checked for valid address and reserved bits.
> 
> Wait a sec...  What about *THIS* series?  Will *THIS* series give us
> oopses when userspace blasts a new XSAVE buffer in with NT_X86_XSTATE?
> 

Fortunately, CET states are supervisor states.  NT_x86_XSTATE has only 
user states.