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Subject[PATCH] x86/asm/entry: Remove user_mode_ignore_vm86()

* Ingo Molnar <mingo@kernel.org> wrote:

> So what the function name wanted to express is something like this:
>
> if (user_mode_vm86_mode_already_checked_so_this_is_marginally_faster_but_dont_use_it_otherwise_because_that_would_be_a_roothole())
> {
> ...
> }
>
> but that name was considered somewhat long.

So how about doing the patch below?

Thanks,

Ingo

===================================>
From 6677d6f073cfda7f1036eb06d13faaad5c6742cc Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Sun, 29 Mar 2015 09:10:08 +0200
Subject: [PATCH] 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.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
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>
Signed-off-by: Ingo Molnar <mingo@kernel.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 d20bae298852..19507ffa5d28 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 56f7e60ad732..e2888a3ad1e3 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 c8eb469a94a4..6751c5c58eec 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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