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, 0 deletions

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 20e5f7ba0e6b..9c289504e680 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -14,6 +14,7 @@
 #include <asm/segment.h>
 #include <asm/irqflags.h>
 #include <asm/asm.h>
+#include <asm/smap.h>
 #include <linux/linkage.h>
 #include <linux/err.h>
 
@@ -146,8 +147,10 @@ ENTRY(ia32_sysenter_target)
         SAVE_ARGS 0,1,0
         /* no need to do an access_ok check here because rbp has been
            32bit zero extended */ 
+        ASM_STAC
 1:      movl    (%rbp),%ebp
         _ASM_EXTABLE(1b,ia32_badarg)
+        ASM_CLAC
         orl     $TS_COMPAT,TI_status+THREAD_INFO(%rsp,RIP-ARGOFFSET)
         testl   $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
         CFI_REMEMBER_STATE
@@ -301,8 +304,10 @@ ENTRY(ia32_cstar_target)
         /* no need to do an access_ok check here because r8 has been
            32bit zero extended */ 
         /* hardware stack frame is complete now */      
+        ASM_STAC
 1:      movl    (%r8),%r9d
         _ASM_EXTABLE(1b,ia32_badarg)
+        ASM_CLAC
         orl     $TS_COMPAT,TI_status+THREAD_INFO(%rsp,RIP-ARGOFFSET)
         testl   $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
         CFI_REMEMBER_STATE
@@ -365,6 +370,7 @@ cstar_tracesys:
 END(ia32_cstar_target)
                                 
 ia32_badarg:
+        ASM_CLAC
         movq $-EFAULT,%rax
         jmp ia32_sysret
         CFI_ENDPROC