x86, smap: Add a header file with macros for STAC/CLAC

The STAC/CLAC instructions are only available with SMAP, but on the
other hand they aren't needed if SMAP is not available, or before we
start to run userspace, so construct them as alternatives which start
out as noops and are enabled by the alternatives mechanism.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348256595-29119-7-git-send-email-hpa@linux.intel.com

2 files changed, 102 insertions, 0 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 8ec3a1aa4abd..5ce86941333f 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1487,6 +1487,17 @@ config ARCH_RANDOM
           If supported, this is a high bandwidth, cryptographically
           secure hardware random number generator.
 
+config X86_SMAP
+        def_bool y
+        prompt "Supervisor Mode Access Prevention" if EXPERT
+        ---help---
+          Supervisor Mode Access Prevention (SMAP) is a security
+          feature in newer Intel processors.  There is a small
+          performance cost if this enabled and turned on; there is
+          also a small increase in the kernel size if this is enabled.
+
+          If unsure, say Y.
+
 config EFI
         bool "EFI runtime service support"
         depends on ACPI
diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
new file mode 100644
index 000000000000..3989c2492eb5
--- /dev/null
+++ b/arch/x86/include/asm/smap.h
@@ -0,0 +1,91 @@
+/*
+ * Supervisor Mode Access Prevention support
+ *
+ * Copyright (C) 2012 Intel Corporation
+ * Author: H. Peter Anvin <hpa@linux.intel.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; version 2
+ * of the License.
+ */
+
+#ifndef _ASM_X86_SMAP_H
+#define _ASM_X86_SMAP_H
+
+#include <linux/stringify.h>
+#include <asm/nops.h>
+#include <asm/cpufeature.h>
+
+/* "Raw" instruction opcodes */
+#define __ASM_CLAC      .byte 0x0f,0x01,0xca
+#define __ASM_STAC      .byte 0x0f,0x01,0xcb
+
+#ifdef __ASSEMBLY__
+
+#include <asm/alternative-asm.h>
+
+#ifdef CONFIG_X86_SMAP
+
+#define ASM_CLAC                                                        \
+        661: ASM_NOP3 ;                                                 \
+        .pushsection .altinstr_replacement, "ax" ;                      \
+        662: __ASM_CLAC ;                                               \
+        .popsection ;                                                   \
+        .pushsection .altinstructions, "a" ;                            \
+        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3 ;       \
+        .popsection
+
+#define ASM_STAC                                                        \
+        661: ASM_NOP3 ;                                                 \
+        .pushsection .altinstr_replacement, "ax" ;                      \
+        662: __ASM_STAC ;                                               \
+        .popsection ;                                                   \
+        .pushsection .altinstructions, "a" ;                            \
+        altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3 ;       \
+        .popsection
+
+#else /* CONFIG_X86_SMAP */
+
+#define ASM_CLAC
+#define ASM_STAC
+
+#endif /* CONFIG_X86_SMAP */
+
+#else /* __ASSEMBLY__ */
+
+#include <asm/alternative.h>
+
+#ifdef CONFIG_X86_SMAP
+
+static inline void clac(void)
+{
+        /* Note: a barrier is implicit in alternative() */
+        alternative(ASM_NOP3, __stringify(__ASM_CLAC), X86_FEATURE_SMAP);
+}
+
+static inline void stac(void)
+{
+        /* Note: a barrier is implicit in alternative() */
+        alternative(ASM_NOP3, __stringify(__ASM_STAC), X86_FEATURE_SMAP);
+}
+
+/* These macros can be used in asm() statements */
+#define ASM_CLAC \
+        ALTERNATIVE(ASM_NOP3, __stringify(__ASM_CLAC), X86_FEATURE_SMAP)
+#define ASM_STAC \
+        ALTERNATIVE(ASM_NOP3, __stringify(__ASM_STAC), X86_FEATURE_SMAP)
+
+#else /* CONFIG_X86_SMAP */
+
+static inline void clac(void) { }
+static inline void stac(void) { }
+
+#define ASM_CLAC
+#define ASM_STAC
+
+#endif /* CONFIG_X86_SMAP */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_SMAP_H */