[tip:x86/asm] x86/asm/entry: Add user_mode_ignore_vm86()

[tip-bot for Andy Lutomirski]

Commit-ID:  a67e7277d01ccfd39b0db5a198c2643cc19dd79c
Gitweb:     http://git.kernel.org/tip/a67e7277d01ccfd39b0db5a198c2643cc19dd79c
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Wed, 18 Mar 2015 18:33:29 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 23 Mar 2015 11:13:36 +0100

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@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brad Spengler <spender@xxxxxxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/202c56ca63823c338af8e2e54948dbe222da6343.1426728647.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 83b874d..4a040f0 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
--
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