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

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Message-ID: <4f8d98ac-69a9-273f-bf23-bb5e06dd0ec9@intel.com>
Date:   Fri, 23 Jul 2021 11:01:24 -0700
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>,
        "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 7/22/2021 2:15 PM, Dave Hansen wrote:
> 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.
> 

That is true.  When this function is called, shadow stack is enabled. 
If get_xsave_addr() returns null, it is possible xstates is messed up. 
Maybe I can update the comments to explain it?

Thanks,
Yu-cheng