linux-kernel - Re: [PATCH v28 26/32] x86/cet/shstk: Introduce shadow stack token setup/verify routines

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Date:   Thu, 22 Jul 2021 14:15:19 -0700
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>,
        Haitao Huang <haitao.huang@...el.com>,
        Rick P Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v28 26/32] x86/cet/shstk: Introduce shadow stack token
 setup/verify routines

On 7/22/21 1:52 PM, Yu-cheng Yu wrote:
> +	if (fpregs_state_valid(fpu, smp_processor_id())) {
> +		rdmsrl(MSR_IA32_PL3_SSP, ssp);
> +	} else {
> +		struct cet_user_state *p;
> +
> +		/*
> +		 * When !fpregs_state_valid() and get_xsave_addr() returns
> +		 * null, XFEAUTRE_CET_USER is in init state.  Shadow stack
> +		 * pointer is null in this case, so return zero.
> +		 */
> +		p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> +		if (p)
> +			ssp = p->user_ssp;
> +	}
> +
> +	fpregs_unlock();

Why are we even calling into this code if shadow stacks might be
disabled?  Seems like we should have just errored out long before
getting here.