linux-kernel - Re: [patch 22/24] x86/speculation: Create PRCTL interface to restrict indirect branch speculation

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Date:   Thu, 22 Nov 2018 13:33:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [patch 22/24] x86/speculation: Create PRCTL interface to
 restrict indirect branch speculation

On Thu, Nov 22, 2018 at 01:26:38PM +0100, Borislav Petkov wrote:
> Perhaps merge the two DISABLE branches to make it obvious what the
> difference between them is:
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 6eac074e3935..28cece3a067b 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -769,7 +769,9 @@ static int indir_branch_prctl_set(struct task_struct *task, unsigned long ctrl)
>  		task_clear_spec_indir_branch_disable(task);
>  		task_update_spec_tif(task, TIF_SPEC_IB, false);
>  		break;
> +
>  	case PR_SPEC_DISABLE:
> +	case PR_SPEC_FORCE_DISABLE:
>  		/*
>  		 * Indirect branch speculation is always allowed when
>  		 * mitigation is force disabled.
> @@ -780,16 +782,11 @@ static int indir_branch_prctl_set(struct task_struct *task, unsigned long ctrl)
>  			return 0;
>  		task_set_spec_indir_branch_disable(task);
>  		task_update_spec_tif(task, TIF_SPEC_IB, true);
> +
> +		if (ctrl == PR_SPEC_FORCE_DISABLE)
> +			task_set_spec_indir_branch_force_disable(task);
>  		break;
> -	case PR_SPEC_FORCE_DISABLE:
> -		if (spectre_v2_app2app == SPECTRE_V2_APP2APP_NONE)
> -			return -EPERM;
> -		if (spectre_v2_app2app == SPECTRE_V2_APP2APP_STRICT)
> -			return 0;
> -		task_set_spec_indir_branch_disable(task);
> -		task_set_spec_indir_branch_force_disable(task);
> -		task_update_spec_tif(task, TIF_SPEC_IB, true);
> -		break;
> +
>  	default:
>  		return -ERANGE;
>  	}

I like that; maybe also do the same to the ssb code, for symmetry.