linux-kernel - [tip:x86/asm] x86/asm/entry: Remove user_mode_ignore

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Date:	Tue, 31 Mar 2015 05:39:22 -0700
From:	tip-bot for Ingo Molnar <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	bp@...e.de, torvalds@...ux-foundation.org, mingo@...nel.org,
	oleg@...hat.com, hpa@...or.com, dvlasenk@...hat.com,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	luto@...capital.net, spender@...ecurity.net, luto@...nel.org,
	bp@...en8.de
Subject: [tip:x86/asm] x86/asm/entry: Remove user_mode_ignore_vm86()

Commit-ID:  55474c48b4726fd3914c1ec47fced0f931729979
Gitweb:     http://git.kernel.org/tip/55474c48b4726fd3914c1ec47fced0f931729979
Author:     Ingo Molnar <mingo@...nel.org>
AuthorDate: Sun, 29 Mar 2015 11:02:34 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 31 Mar 2015 11:45:19 +0200

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@...e.de>
Acked-by: Andy Lutomirski <luto@...nel.org>
Cc: Andrew Lutomirski <luto@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brad Spengler <spender@...ecurity.net>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20150329090233.GA1963@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/include/asm/ptrace.h    | 17 -----------------
 arch/x86/kernel/cpu/perf_event.c |  2 +-
 arch/x86/kernel/traps.c          |  6 +++---
 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;
--
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