x86/asm/entry: Make user_mode() work correctly if regs came from VM86 mode

user_mode() is now identical to user_mode_vm().  Subsequent patches
will change all callers of user_mode_vm() to user_mode() and then
delete user_mode_vm().

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/0dd03eacb5f0a2b5ba0240de25347a31b493c289.1426728647.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/ptrace.h

@@ -96,11 +96,13 @@ static inline unsigned long regs_return_value(struct pt_regs *regs)
 }
 
 /*
- * user_mode_vm(regs) determines whether a register set came from user mode.
- * This is true if V8086 mode was enabled OR if the register set was from
- * protected mode with RPL-3 CS value.  This tricky test checks that with
- * one comparison.  Many places in the kernel can bypass this full check
- * if they have already ruled out V8086 mode, so user_mode(regs) can be used.
+ * user_mode(regs) determines whether a register set came from user
+ * mode.  On x86_32, this is true if V8086 mode was enabled OR if the
+ * register set was from protected mode with RPL-3 CS value.  This
+ * tricky test checks that with one comparison.
+ *
+ * On x86_64, vm86 mode is mercifully nonexistent, and we don't need
+ * the extra check.
  */
 static inline int user_mode(struct pt_regs *regs)
 {
@@ -113,12 +115,7 @@ static inline int user_mode(struct pt_regs *regs)
 
 static inline int user_mode_vm(struct pt_regs *regs)
 {
-#ifdef CONFIG_X86_32
-	return ((regs->cs & SEGMENT_RPL_MASK) | (regs->flags & X86_VM_MASK)) >=
-		USER_RPL;
-#else
 	return user_mode(regs);
-#endif
 }
 
 /*