linux-kernel - Re: [PATCH] x86/bugs: Default retbleed to =stuff when retpoline is auto enabled

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Date: Fri, 9 Feb 2024 10:56:25 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	antonio.gomez.iglesias@...ux.intel.com,
	alyssa.milburn@...ux.intel.com, andrew.cooper3@...rix.com,
	linux-kernel@...r.kernel.org,
	Alyssa Milburn <alyssa.milburn@...el.com>
Subject: Re: [PATCH] x86/bugs: Default retbleed to =stuff when retpoline is
 auto enabled

On Thu, Feb 08, 2024 at 05:12:15PM -0800, Pawan Gupta wrote:
> @@ -1025,10 +1041,17 @@ static void __init retbleed_select_mitigation(void)
>  				retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
>  			else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY) && boot_cpu_has(X86_FEATURE_IBPB))
>  				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
> +		} else if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
> +			   spectre_v2_parse_cmdline() == SPECTRE_V2_CMD_AUTO &&

spectre_v2_parse_cmdline() has side effects (printks) and shouldn't be
called twice.

And what's the point of checking spectre_v2= anyway?  Shouldn't retbleed
be mitigated by default, independently of whatever the user may have
specified for Spectre v2?

> +			   spectre_v2_enabled == SPECTRE_V2_RETPOLINE) {
> +			if (IS_ENABLED(CONFIG_CALL_DEPTH_TRACKING))
> +				retbleed_mitigation = RETBLEED_MITIGATION_STUFF;
> +			else
> +				pr_err("WARNING: Retpoline enabled, but kernel not compiled with CALL_DEPTH_TRACKING.\n");

If retbleed is vulnerable then the sysfs file should show that.

Also, I think this pr_err() is redundant with RETBLEED_INTEL_MSG and can
be removed.

-- 
Josh