cpu/speculation: Add 'mitigations=' cmdline option

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@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Jiri Kosina <jkosina@suse.cz> (on x86)
Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux-s390@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-arch@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tyler Hicks <tyhicks@canonical.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Phil Auld <pauld@redhat.com>
Link: https://lkml.kernel.org/r/b07a8ef9b7c5055c3a4637c87d07c296d5016fe0.1555085500.git.jpoimboe@redhat.com

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 858b6c0b9a15ae71853645013c718dc0398804d4..720ffa9c4e04b6ecfd468184f1284938f0cb6393 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2513,6 +2513,30 @@
                        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.
+
+                       auto,nosmt
+                               Mitigate all CPU vulnerabilities, disabling SMT
+                               if needed.  This is for users who always want to
+                               be fully mitigated, even if it means losing SMT.
+
        mminit_loglevel=
                        [KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this
                        parameter allows control of the logging verbosity for

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 5041357d0297afdce8b5605970947b34f85e7205..2d9c6f4b78f5804971e4292771e0247bd3c629fa 100644 (file)
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -187,4 +187,28 @@ static inline void cpu_smt_disable(bool force) { }
 static inline void cpu_smt_check_topology(void) { }
 #endif
 
+/*
+ * These are used for a global "mitigations=" cmdline option for toggling
+ * optional CPU mitigations.
+ */
+enum cpu_mitigations {
+       CPU_MITIGATIONS_OFF,
+       CPU_MITIGATIONS_AUTO,
+       CPU_MITIGATIONS_AUTO_NOSMT,
+};
+
+extern enum cpu_mitigations cpu_mitigations;
+
+/* mitigations=off */
+static inline bool cpu_mitigations_off(void)
+{
+       return cpu_mitigations == CPU_MITIGATIONS_OFF;
+}
+
+/* mitigations=auto,nosmt */
+static inline bool cpu_mitigations_auto_nosmt(void)
+{
+       return cpu_mitigations == CPU_MITIGATIONS_AUTO_NOSMT;
+}
+
 #endif /* _LINUX_CPU_H_ */

diff --git a/kernel/cpu.c b/kernel/cpu.c
index d1c6d152da890dab7b8dab9530dbd1550d85fef6..e70a90634b41cd57de7a6fed4d3ff40d24055590 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2279,3 +2279,18 @@ void __init boot_cpu_hotplug_init(void)
 #endif
        this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
 }
+
+enum cpu_mitigations cpu_mitigations __ro_after_init = 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;
+       else if (!strcmp(arg, "auto,nosmt"))
+               cpu_mitigations = CPU_MITIGATIONS_AUTO_NOSMT;
+
+       return 0;
+}
+early_param("mitigations", mitigations_parse_cmdline);