x86/asm/entry: Remove user_mode_ignore_vm86()

user_mode_ignore_vm86() can be used instead of user_mode(), in
places where we have already done a v8086_mode() security
check of ptregs.

But doing this check in the wrong place would be a bug that
could result in security problems, and also the naming still
isn't very clear.

Furthermore, it only affects 32-bit kernels, while most
development happens on 64-bit kernels.

If we replace them with user_mode() checks then the cost is only
a very minor increase in various slowpaths:

   text             data   bss     dec              hex    filename
   10573391         703562 1753042 13029995         c6d26b vmlinux.o.before
   10573423         703562 1753042 13030027         c6d28b vmlinux.o.after

So lets get rid of this distinction once and for all.

Acked-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
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: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150329090233.GA1963@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 4 insertions, 21 deletions

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index d20bae2..19507ff 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -113,23 +113,6 @@ static inline int user_mode(struct pt_regs *regs)
 #endif
 }
 
-/*
- * This is the fastest way to check whether regs come from user space.
- * It is unsafe if regs might come from vm86 mode, though -- in vm86
- * mode, all bits of CS and SS are completely under the user's control.
- * The CPU considers vm86 mode to be CPL 3 regardless of CS and SS.
- *
- * Do NOT use this function unless you have already ruled out the
- * possibility that regs came from vm86 mode.
- *
- * We check for RPL != 0 instead of RPL == 3 because we don't use rings
- * 1 or 2 and this is more efficient.
- */
-static inline int user_mode_ignore_vm86(struct pt_regs *regs)
-{
-	return (regs->cs & SEGMENT_RPL_MASK) != 0;
-}
-
 static inline int v8086_mode(struct pt_regs *regs)
 {
 #ifdef CONFIG_X86_32
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 56f7e60..e2888a3 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -2159,7 +2159,7 @@ static unsigned long code_segment_base(struct pt_regs *regs)
 	if (regs->flags & X86_VM_MASK)
 		return 0x10 * regs->cs;
 
-	if (user_mode_ignore_vm86(regs) && regs->cs != __USER_CS)
+	if (user_mode(regs) && regs->cs != __USER_CS)
 		return get_segment_base(regs->cs);
 #else
 	if (user_mode(regs) && !user_64bit_mode(regs) &&
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c8eb469..6751c5c 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -207,7 +207,7 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, char *str,
 		return -1;
 	}
 
-	if (!user_mode_ignore_vm86(regs)) {
+	if (!user_mode(regs)) {
 		if (!fixup_exception(regs)) {
 			tsk->thread.error_code = error_code;
 			tsk->thread.trap_nr = trapnr;
@@ -468,7 +468,7 @@ do_general_protection(struct pt_regs *regs, long error_code)
 	}
 
 	tsk = current;
-	if (!user_mode_ignore_vm86(regs)) {
+	if (!user_mode(regs)) {
 		if (fixup_exception(regs))
 			goto exit;
 
@@ -685,7 +685,7 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	 * We already checked v86 mode above, so we can check for kernel mode
 	 * by just checking the CPL of CS.
 	 */
-	if ((dr6 & DR_STEP) && !user_mode_ignore_vm86(regs)) {
+	if ((dr6 & DR_STEP) && !user_mode(regs)) {
 		tsk->thread.debugreg6 &= ~DR_STEP;
 		set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
 		regs->flags &= ~X86_EFLAGS_TF;