linux-kernel - Re: [PATCH 14/29] x86/ibt: Add IBT feature, MSR and #CP handling

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Date:   Fri, 18 Feb 2022 19:31:38 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alyssa.milburn@...el.com" <alyssa.milburn@...el.com>
Subject: Re: [PATCH 14/29] x86/ibt: Add IBT feature, MSR and #CP handling

On 18/02/2022 16:49, Peter Zijlstra wrote:
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -592,6 +593,27 @@ static __init int setup_disable_pku(char
>  __setup("nopku", setup_disable_pku);
>  #endif /* CONFIG_X86_64 */
>  
> +static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> +{
> +	u64 msr;
> +
> +	if (!IS_ENABLED(CONFIG_X86_IBT) ||
> +	    !cpu_feature_enabled(X86_FEATURE_IBT))
> +		return;
> +
> +	cr4_set_bits(X86_CR4_CET);
> +
> +	rdmsrl(MSR_IA32_S_CET, msr);
> +	if (cpu_feature_enabled(X86_FEATURE_IBT))
> +		msr |= CET_ENDBR_EN;
> +	wrmsrl(MSR_IA32_S_CET, msr);

So something I learnt the hard way with shstk is that you really want to
disable S_CET before heading into purgatory.

I've got no idea what's going to result from UEFI finally getting CET
support.  However, clearing out the other IBT settings is probably a
wise move.

In particular, if there was a stale legacy bitmap pointer, then
ibt_selftest() could take #PF ahead of #CP.

~Andrew