From a67e7277d01ccfd39b0db5a198c2643cc19dd79c Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@kernel.org>
Date: Wed, 18 Mar 2015 18:33:29 -0700
Subject: [PATCH] x86/asm/entry: Add user_mode_ignore_vm86()
user_mode() is dangerous and user_mode_vm() has a confusing name.
Add user_mode_ignore_vm86() (equivalent to current user_mode()).
We'll change the small number of legitimate users of user_mode()
to user_mode_ignore_vm86().
Inspired by grsec, although this works rather differently.
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/202c56ca63823c338af8e2e54948dbe222da6343.1426728647.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/ptrace.h | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 83b874da2762..4a040f0078f2 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -121,6 +121,23 @@ static inline int user_mode_vm(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