linux-kernel - [PATCH v2 2/2] x86/bugs: spectre user default must depend on MITIGATION_SPECTRE

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Message-Id: <20241031-x86_bugs_last_v2-v2-2-b7ff1dab840e@debian.org>
Date: Thu, 31 Oct 2024 04:06:17 -0700
From: Breno Leitao <leitao@...ian.org>
To: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
 Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
 Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, 
 Jonathan Corbet <corbet@....net>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
 Breno Leitao <leitao@...ian.org>
Subject: [PATCH v2 2/2] x86/bugs: spectre user default must depend on
 MITIGATION_SPECTRE_V2

Change the default value of spectre v2 in user mode to respect the
CONFIG_MITIGATION_SPECTRE_V2 config option.

Currently, user mode spectre v2 is set to auto
(SPECTRE_V2_USER_CMD_AUTO) by default, even if
CONFIG_MITIGATION_SPECTRE_V2 is disabled.

Set the spectre_v2 value to auto (SPECTRE_V2_USER_CMD_AUTO) if the
Spectre v2 config (CONFIG_MITIGATION_SPECTRE_V2) is enabled, otherwise
set the value to none (SPECTRE_V2_USER_CMD_NONE).

Important to say the command line argument "spectre_v2_user" overwrites
the default value in both cases.

When CONFIG_MITIGATION_SPECTRE_V2 is not set, users have the flexibility
to opt-in for specific mitigations independently. In this scenario,
setting spectre_v2= will not enable spectre_v2_user=, and command line
options spectre_v2_user and spectre_v2 are independent when
CONFIG_MITIGATION_SPECTRE_V2=n.

Signed-off-by: Breno Leitao <leitao@...ian.org>
Acked-by: Josh Poimboeuf <jpoimboe@...nel.org>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  2 ++
 arch/x86/kernel/cpu/bugs.c                      | 10 +++++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1518343bbe2237f1d577df5656339d6224b769be..f8bc02cd10ec3012c478e12e90bad85d9f70d9ed 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6241,6 +6241,8 @@

 			Selecting 'on' will also enable the mitigation
 			against user space to user space task attacks.
+			Selecting specific mitigation does not force enable
+			user mitigations.

 			Selecting 'off' will disable both the kernel and
 			the user space protections.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index ceaa5f0c742025a62aa564aaa9cdfa1fe86e681c..236c691c3fd2ac7d41c494d5c7fbc844d11ae82f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1294,9 +1294,13 @@ 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;
@@ -1309,7 +1313,7 @@ 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;

 	for (i = 0; i < ARRAY_SIZE(v2_user_options); i++) {
 		if (match_option(arg, ret, v2_user_options[i].option)) {
@@ -1319,8 +1323,8 @@ spectre_v2_parse_user_cmdline(void)
 		}
 	}

-	pr_err("Unknown user space protection option (%s). Switching to AUTO select\n", arg);
-	return SPECTRE_V2_USER_CMD_AUTO;
+	pr_err("Unknown user space protection option (%s). Switching to default\n", arg);
+	return mode;
 }

 static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)

-- 
2.43.5