[PATCH AUTOSEL 5.4 37/79] x86/bugs: Make spectre user default depend on MITIGATION_SPECTRE_V2

[Sasha Levin <sashal@kernel.org>]

From: Breno Leitao <leitao@debian.org>

[ Upstream commit 98fdaeb296f51ef08e727a7cc72e5b5c864c4f4d ]

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@debian.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Kaplan <David.Kaplan@amd.com>
Link: https://lore.kernel.org/r/20241031-x86_bugs_last_v2-v2-2-b7ff1dab840e@debian.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 9975dcab99c35..6d9acc3f977b3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4600,6 +4600,8 @@
 
 			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 4f803aed2ef0e..0f523ebfbabf6 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1050,9 +1050,13 @@ 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;
@@ -1065,7 +1069,7 @@ 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)) {
@@ -1075,8 +1079,8 @@ 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_eibrs_mode(enum spectre_v2_mitigation mode)
-- 
2.39.5