linux-kernel - Re: [PATCH 2/2] x86/bugs: spectre user default must depend on MITIGATION_SPECTRE

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Message-ID: <20241028143453.govo3bnbayc7rqjj@desk>
Date: Mon, 28 Oct 2024 07:34:53 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Breno Leitao <leitao@...ian.org>
Cc: bp@...en8.de, x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, kernel-team@...a.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/bugs: spectre user default must depend on
 MITIGATION_SPECTRE_V2

On Tue, Oct 15, 2024 at 03:51:06AM -0700, Breno Leitao wrote:
> @@ -1277,9 +1277,13 @@ static __ro_after_init enum spectre_v2_mitigation_cmd spectre_v2_cmd;
>  static enum spectre_v2_user_cmd __init
>  spectre_v2_parse_user_cmdline(void)
>  {
> +	enum spectre_v2_user_cmd mode;
>  	char arg[20];
>  	int ret, i;
>  
> +	mode = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?
> +		SPECTRE_V2_USER_CMD_AUTO : SPECTRE_V2_USER_CMD_NONE;
> +
>  	switch (spectre_v2_cmd) {
>  	case SPECTRE_V2_CMD_NONE:
>  		return SPECTRE_V2_USER_CMD_NONE;
> @@ -1292,7 +1296,7 @@ spectre_v2_parse_user_cmdline(void)
>  	ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
>  				  arg, sizeof(arg));
>  	if (ret < 0)
> -		return SPECTRE_V2_USER_CMD_AUTO;
> +		return mode;

This doesn't look right to me, spectre_v2=eibrs|retpoline... will override
CONFIG_MITIGATION_SPECTRE_V2=n and enable the kernel mitigation, but the
user mitigation will stay disabled. If this is the intention it should be
clearly documented that enabling kernel mitigation does not enable user
mitigation. And an explicit spectre_v2_user= is required to enable user
mitigation.