linux-kernel - Re: [PATCH v3 04/37] x86/cpufeatures: Enable CET CR4 bit for shadow stack

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Message-ID: <Y2lHxb5BnbQi499s@zn.tnic>
Date:   Mon, 7 Nov 2022 19:00:37 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     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>,
        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>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v3 04/37] x86/cpufeatures: Enable CET CR4 bit for shadow
 stack

On Fri, Nov 04, 2022 at 03:35:31PM -0700, Rick Edgecombe wrote:
>  static __always_inline void setup_cet(struct cpuinfo_x86 *c)
>  {
> -	u64 msr = CET_ENDBR_EN;
> +	bool kernel_ibt = HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT);
> +	bool user_shstk;
> +	u64 msr = 0;
>  
> -	if (!HAS_KERNEL_IBT ||
> -	    !cpu_feature_enabled(X86_FEATURE_IBT))
> +	/*
> +	 * Enable user shadow stack only if the Linux defined user shadow stack
> +	 * cap was not cleared by command line.
> +	 */
> +	user_shstk = cpu_feature_enabled(X86_FEATURE_SHSTK) &&
> +		     IS_ENABLED(CONFIG_X86_USER_SHADOW_STACK) &&
> +		     !test_bit(X86_FEATURE_USER_SHSTK, (unsigned long *)cpu_caps_cleared);

Huh, why poke at cpu_caps_cleared? 

Look below:

> +	if (!kernel_ibt && !user_shstk)
>  		return;
>  
> +	if (user_shstk)
> +		set_cpu_cap(c, X86_FEATURE_USER_SHSTK);
> +
> +	if (kernel_ibt)
> +		msr = CET_ENDBR_EN;
> +
>  	wrmsrl(MSR_IA32_S_CET, msr);
>  	cr4_set_bits(X86_CR4_CET);
>  
> -	if (!ibt_selftest()) {
> +	if (kernel_ibt && !ibt_selftest()) {
>  		pr_err("IBT selftest: Failed!\n");
>  		setup_clear_cpu_cap(X86_FEATURE_IBT);
>  		return;
>  	}
>  }
> +#else /* CONFIG_X86_CET */
> +static inline void setup_cet(struct cpuinfo_x86 *c) {}
> +#endif
>  
>  __noendbr void cet_disable(void)
>  {
> -	if (cpu_feature_enabled(X86_FEATURE_IBT))
> -		wrmsrl(MSR_IA32_S_CET, 0);
> +	if (!(cpu_feature_enabled(X86_FEATURE_IBT) ||
> +	      cpu_feature_enabled(X86_FEATURE_SHSTK)))
> +		return;
> +
> +	wrmsrl(MSR_IA32_S_CET, 0);
> +	wrmsrl(MSR_IA32_U_CET, 0);

Here you need to do

	setup_clear_cpu_cap(X86_FEATURE_IBT);
	setup_clear_cpu_cap(X86_FEATURE_SHSTK);

and then the cpu_feature_enabled() test above alone should suffice.

But, before you do that, I'd like to ask you to update your patchset
ontop of tip/master because the conflicts are getting non-trivial. This
one doesn't even want to apply with a large fuzz:

$ patch -p1 --dry-run -F20 -i /tmp/new
checking file arch/x86/kernel/cpu/common.c
Hunk #1 FAILED at 596.
1 out of 1 hunk FAILED

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette