linux-kernel - Re: [PATCH v3] x86/bugs: Only harden syscalls when needed

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Date: Wed, 17 Apr 2024 16:14:26 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Alexandre Chartre <alexandre.chartre@...cle.com>,
 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Nikolay Borisov <nik.borisov@...e.com>, KP Singh <kpsingh@...nel.org>,
 Waiman Long <longman@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3] x86/bugs: Only harden syscalls when needed

On 17/04/2024 12:02 am, Josh Poimboeuf wrote:
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index ca295b0c1eee..dcb97cc2758f 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1678,6 +1687,21 @@ static void __init spectre_v2_select_mitigation(void)
>  	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
>  	enum spectre_v2_mitigation mode = SPECTRE_V2_NONE;
>  
> +	/*
> +	 * X86_FEATURE_INDIRECT_SAFE indicates whether indirect calls can be
> +	 * considered safe.  That means either:
> +	 *
> +	 *   - the CPU isn't vulnerable to Spectre v2 or its variants;
> +	 *
> +	 *   - a hardware mitigation is in place (e.g., IBRS, BHI_DIS_S); or
> +	 *
> +	 *   - the user turned off mitigations altogether.
> +	 *
> +	 * Assume innocence until proven guilty: set the cap bit now, then
> +	 * clear it later if/when needed.
> +	 */
> +	setup_force_cpu_cap(X86_FEATURE_INDIRECT_SAFE);

Following on from the (re)discovery that X86_FEATURE_RETPOLINE is a poor
name given what it *actually* does, can I recommend s/SAFE/OK/ here?

This flag really is "do I want indirect branches or not", which - as
noted here - is more than just a judgement of whether indirect branches
are "safe".

~Andrew