linux-kernel - [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86 architectures

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Date: Tue, 16 Apr 2024 17:15:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Peter Zijlstra <peterz@...radead.org>
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Stephen Rothwell <sfr@...b.auug.org.au>, Michael Ellerman <mpe@...erman.id.au>, 
	Geert Uytterhoeven <geert@...ux-m68k.org>, Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH 1/2] cpu: Re-enable CPU mitigations by default for !X86 architectures

Add a generic Kconfig, CPU_MITIGATIONS, to control whether or not CPU
mitigations are enabled by default, and force it on for all architectures
except x86.  A recent commit to turn mitigations off by default if
SPECULATION_MITIGATIONS=n kinda sorta missed that "cpu_mitigations" is
completely generic, where as SPECULATION_MITIGATIONS is x86 specific.

Alternatively, SPECULATION_MITIGATIONS could simply be defined in common
code, but that creates weirdness for x86 because SPECULATION_MITIGATIONS
ends up being defined twice, and the default behavior would likely depend
on the arbitrary include order (if the two definitions diverged).

Ideally, CPU_MITIGATIONS would be unconditionally on by default for all
architectures, and manually turned off, but there is no way to unselect a
Kconfig.

Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Reported-by: Michael Ellerman <mpe@...erman.id.au>
Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
Closes: https://lkml.kernel.org/r/20240413115324.53303a68%40canb.auug.org.au
Fixes: f337a6a21e2f ("x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n")
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/Kconfig     | 1 +
 drivers/base/Kconfig | 3 +++
 kernel/cpu.c         | 4 ++--
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4474bf32d0a4..a0eca6313276 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2490,6 +2490,7 @@ config PREFIX_SYMBOLS
 
 menuconfig SPECULATION_MITIGATIONS
 	bool "Mitigations for speculative execution vulnerabilities"
+	select CPU_MITIGATIONS
 	default y
 	help
 	  Say Y here to enable options which enable mitigations for
diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 2b8fd6bb7da0..dab19f15fa57 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -191,6 +191,9 @@ config GENERIC_CPU_AUTOPROBE
 config GENERIC_CPU_VULNERABILITIES
 	bool
 
+config CPU_MITIGATIONS
+	def_bool !X86
+
 config SOC_BUS
 	bool
 	select GLOB
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 07ad53b7f119..bb0ff275fb46 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3207,8 +3207,8 @@ enum cpu_mitigations {
 };
 
 static enum cpu_mitigations cpu_mitigations __ro_after_init =
-	IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
-						     CPU_MITIGATIONS_OFF;
+	IS_ENABLED(CONFIG_CPU_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
+					     CPU_MITIGATIONS_OFF;
 
 static int __init mitigations_parse_cmdline(char *arg)
 {
-- 
2.44.0.683.g7961c838ac-goog