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

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Date:   Fri, 29 Jan 2021 13:00:19 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...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: [NEEDS-REVIEW] [PATCH v18 05/25] x86/fpu/xstate: Introduce CET
 MSR and XSAVES supervisor states

On 1/27/21 1:25 PM, Yu-cheng Yu wrote:
> @@ -135,6 +135,8 @@ enum xfeature {
>  #define XFEATURE_MASK_PT		(1 << XFEATURE_PT_UNIMPLEMENTED_SO_FAR)
>  #define XFEATURE_MASK_PKRU		(1 << XFEATURE_PKRU)
>  #define XFEATURE_MASK_PASID		(1 << XFEATURE_PASID)
> +#define XFEATURE_MASK_CET_USER		(1 << XFEATURE_CET_USER)
> +#define XFEATURE_MASK_CET_KERNEL	(1 << XFEATURE_CET_KERNEL)
>  #define XFEATURE_MASK_LBR		(1 << XFEATURE_LBR)
>  
>  #define XFEATURE_MASK_FPSSE		(XFEATURE_MASK_FP | XFEATURE_MASK_SSE)
> @@ -237,6 +239,23 @@ struct pkru_state {
>  	u32				pad;
>  } __packed;
>  
> +/*
> + * State component 11 is Control-flow Enforcement user states
> + */
> +struct cet_user_state {
> +	u64 user_cet;			/* user control-flow settings */
> +	u64 user_ssp;			/* user shadow stack pointer */
> +};

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?