Commit 98af8452 authored Apr 12, 2019 by Josh Poimboeuf Committed by Thomas Gleixner Apr 17, 2019

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

parent 1c163f4c

Documentation/admin-guide/kernel-parameters.txt

+24 −0

Original line number	Diff line number	Diff line
		@@ -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

include/linux/cpu.h

+24 −0

Original line number	Diff line number	Diff line
		@@ -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_ */

kernel/cpu.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -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);

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