linux-kernel - Re: [PATCH 03/19] x86/cpufeatures: Enable CET CR4 bit for shadow stack

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Message-ID: <YqsFIRDPvaEKMqIh@hirez.programming.kicks-ass.net>
Date:   Thu, 16 Jun 2022 12:25:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, seanjc@...gle.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com, Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 03/19] x86/cpufeatures: Enable CET CR4 bit for shadow
 stack

On Thu, Jun 16, 2022 at 04:46:27AM -0400, Yang Weijiang wrote:

>  static __always_inline void setup_cet(struct cpuinfo_x86 *c)
>  {
> +	bool kernel_ibt = HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT);
>  	u64 msr = CET_ENDBR_EN;
>  
> +	if (kernel_ibt)
> +		wrmsrl(MSR_IA32_S_CET, msr);
>  
> +	if (kernel_ibt || cpu_feature_enabled(X86_FEATURE_SHSTK))
> +		cr4_set_bits(X86_CR4_CET);

Does flipping the CR4 and S_CET MSR write not result in simpler code?

>  
> +	if (kernel_ibt && !ibt_selftest()) {
>  		pr_err("IBT selftest: Failed!\n");
>  		setup_clear_cpu_cap(X86_FEATURE_IBT);

Looking at this error path; I think I forgot to clear S_CET here.

>  		return;
>  	}
>  }