x86, smap: Add STAC and CLAC instructions to control user space access

When Supervisor Mode Access Prevention (SMAP) is enabled, access to
userspace from the kernel is controlled by the AC flag.  To make the
performance of manipulating that flag acceptable, there are two new
instructions, STAC and CLAC, to set and clear it.

This patch adds those instructions, via alternative(), when the SMAP
feature is enabled.  It also adds X86_EFLAGS_AC unconditionally to the
SYSCALL entry mask; there is simply no reason to make that one
conditional.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/r/1348256595-29119-9-git-send-email-hpa@linux.intel.com

1 files changed, 6 insertions, 4 deletions

diff --git a/arch/x86/include/asm/xsave.h b/arch/x86/include/asm/xsave.h
index 8a1b6f9b594a..2a923bd54341 100644
--- a/arch/x86/include/asm/xsave.h
+++ b/arch/x86/include/asm/xsave.h
@@ -74,8 +74,9 @@ static inline int xsave_user(struct xsave_struct __user *buf)
         if (unlikely(err))
                 return -EFAULT;
 
-        __asm__ __volatile__("1: .byte " REX_PREFIX "0x0f,0xae,0x27\n"
-                             "2:\n"
+        __asm__ __volatile__(ASM_STAC "\n"
+                             "1: .byte " REX_PREFIX "0x0f,0xae,0x27\n"
+                             "2: " ASM_CLAC "\n"
                              ".section .fixup,\"ax\"\n"
                              "3:  movl $-1,%[err]\n"
                              "    jmp  2b\n"
@@ -97,8 +98,9 @@ static inline int xrestore_user(struct xsave_struct __user *buf, u64 mask)
         u32 lmask = mask;
         u32 hmask = mask >> 32;
 
-        __asm__ __volatile__("1: .byte " REX_PREFIX "0x0f,0xae,0x2f\n"
-                             "2:\n"
+        __asm__ __volatile__(ASM_STAC "\n"
+                             "1: .byte " REX_PREFIX "0x0f,0xae,0x2f\n"
+                             "2: " ASM_CLAC "\n"
                              ".section .fixup,\"ax\"\n"
                              "3:  movl $-1,%[err]\n"
                              "    jmp  2b\n"