[PATCH 4.4 247/266] cpu/speculation: Add mitigations= cmdline option

[Greg Kroah-Hartman]

From: Josh Poimboeuf <jpoimboe at redhat.com>

commit 98af8452945c55652de68536afdde3b520fec429 upstream.

Keeping track of the number of mitigations for all the CPU speculation
bugs has become overwhelming for many users.  It's getting more and more
complicated to decide which mitigations are needed for a given
architecture.  Complicating matters is the fact that each arch tends to
have its own custom way to mitigate the same vulnerability.

Most users fall into a few basic categories:

a) they want all mitigations off;

b) they want all reasonable mitigations on, with SMT enabled even if
   it's vulnerable; or

c) they want all reasonable mitigations on, with SMT disabled if
   vulnerable.

Define a set of curated, arch-independent options, each of which is an
aggregation of existing options:

- mitigations=off: Disable all mitigations.

- mitigations=auto: [default] Enable all the default mitigations, but
  leave SMT enabled, even if it's vulnerable.

- mitigations=auto,nosmt: Enable all the default mitigations, disabling
  SMT if needed by a mitigation.

Currently, these options are placeholders which don't actually do
anything.  They will be fleshed out in upcoming patches.

Signed-off-by: Josh Poimboeuf <jpoimboe at redhat.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Tested-by: Jiri Kosina <jkosina at suse.cz> (on x86)
Reviewed-by: Jiri Kosina <jkosina at suse.cz>
Cc: Borislav Petkov <bp at alien8.de>
Cc: "H . Peter Anvin" <hpa at zytor.com>
Cc: Andy Lutomirski <luto at kernel.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Jiri Kosina <jikos at kernel.org>
Cc: Waiman Long <longman at redhat.com>
Cc: Andrea Arcangeli <aarcange at redhat.com>
Cc: Jon Masters <jcm at redhat.com>
Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Cc: Paul Mackerras <paulus at samba.org>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: linuxppc-dev at lists.ozlabs.org
Cc: Martin Schwidefsky <schwidefsky at de.ibm.com>
Cc: Heiko Carstens <heiko.carstens at de.ibm.com>
Cc: linux-s390 at vger.kernel.org
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Will Deacon <will.deacon at arm.com>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-arch at vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Tyler Hicks <tyhicks at canonical.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Randy Dunlap <rdunlap at infradead.org>
Cc: Steven Price <steven.price at arm.com>
Cc: Phil Auld <pauld at redhat.com>
Link: https://lkml.kernel.org/r/b07a8ef9b7c5055c3a4637c87d07c296d5016fe0.1555085500.git.jpoimboe@redhat.com
[bwh: Backported to 4.4:
 - Drop the auto,nosmt option which we can't support
 - Adjust filename]
Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
---
 Documentation/kernel-parameters.txt |   19 +++++++++++++++++++
 include/linux/cpu.h                 |   17 +++++++++++++++++
 kernel/cpu.c                        |   13 +++++++++++++
 3 files changed, 49 insertions(+)

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2173,6 +2173,25 @@ bytes respectively. Such letter suffixes
 			in the "bleeding edge" mini2440 support kernel at
 			http://repo.or.cz/w/linux-2.6/mini2440.git
 
+	mitigations=
+			Control optional mitigations for CPU vulnerabilities.
+			This is a set of curated, arch-independent options, each
+			of which is an aggregation of existing arch-specific
+			options.
+
+			off
+				Disable all optional CPU mitigations.  This
+				improves system performance, but it may also
+				expose users to several CPU vulnerabilities.
+
+			auto (default)
+				Mitigate all CPU vulnerabilities, but leave SMT
+				enabled, even if it's vulnerable.  This is for
+				users who don't want to be surprised by SMT
+				getting disabled across kernel upgrades, or who
+				have other ways of avoiding SMT-based attacks.
+				This is the default behavior.
+
 	mminit_loglevel=
 			[KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this
 			parameter allows control of the logging verbosity for
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -296,4 +296,21 @@ bool cpu_wait_death(unsigned int cpu, in
 bool cpu_report_death(void);
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
+/*
+ * These are used for a global "mitigations=" cmdline option for toggling
+ * optional CPU mitigations.
+ */
+enum cpu_mitigations {
+	CPU_MITIGATIONS_OFF,
+	CPU_MITIGATIONS_AUTO,
+};
+
+extern enum cpu_mitigations cpu_mitigations;
+
+/* mitigations=off */
+static inline bool cpu_mitigations_off(void)
+{
+	return cpu_mitigations == CPU_MITIGATIONS_OFF;
+}
+
 #endif /* _LINUX_CPU_H_ */
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -842,3 +842,16 @@ void init_cpu_online(const struct cpumas
 {
 	cpumask_copy(to_cpumask(cpu_online_bits), src);
 }
+
+enum cpu_mitigations cpu_mitigations = CPU_MITIGATIONS_AUTO;
+
+static int __init mitigations_parse_cmdline(char *arg)
+{
+	if (!strcmp(arg, "off"))
+		cpu_mitigations = CPU_MITIGATIONS_OFF;
+	else if (!strcmp(arg, "auto"))
+		cpu_mitigations = CPU_MITIGATIONS_AUTO;
+
+	return 0;
+}
+early_param("mitigations", mitigations_parse_cmdline);