x86/entry/vsyscall: Add CONFIG to control default

Most modern systems can run with vsyscall=none. In an effort to
provide a way for build-time defaults to lack legacy settings,
this adds a new CONFIG to select the type of vsyscall mapping to
use, similar to the existing "vsyscall" command line parameter.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150813005519.GA11696@www.outflux.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Kconfig

@@ -2042,6 +2042,55 @@ config COMPAT_VDSO
 	  If unsure, say N: if you are compiling your own kernel, you
 	  are unlikely to be using a buggy version of glibc.
 
+choice
+	prompt "vsyscall table for legacy applications"
+	depends on X86_64
+	default LEGACY_VSYSCALL_EMULATE
+	help
+	  Legacy user code that does not know how to find the vDSO expects
+	  to be able to issue three syscalls by calling fixed addresses in
+	  kernel space. Since this location is not randomized with ASLR,
+	  it can be used to assist security vulnerability exploitation.
+
+	  This setting can be changed at boot time via the kernel command
+	  line parameter vsyscall=[native|emulate|none].
+
+	  On a system with recent enough glibc (2.14 or newer) and no
+	  static binaries, you can say None without a performance penalty
+	  to improve security.
+
+	  If unsure, select "Emulate".
+
+	config LEGACY_VSYSCALL_NATIVE
+		bool "Native"
+		help
+		  Actual executable code is located in the fixed vsyscall
+		  address mapping, implementing time() efficiently. Since
+		  this makes the mapping executable, it can be used during
+		  security vulnerability exploitation (traditionally as
+		  ROP gadgets). This configuration is not recommended.
+
+	config LEGACY_VSYSCALL_EMULATE
+		bool "Emulate"
+		help
+		  The kernel traps and emulates calls into the fixed
+		  vsyscall address mapping. This makes the mapping
+		  non-executable, but it still contains known contents,
+		  which could be used in certain rare security vulnerability
+		  exploits. This configuration is recommended when userspace
+		  still uses the vsyscall area.
+
+	config LEGACY_VSYSCALL_NONE
+		bool "None"
+		help
+		  There will be no vsyscall mapping at all. This will
+		  eliminate any risk of ASLR bypass due to the vsyscall
+		  fixed address mapping. Attempts to use the vsyscalls
+		  will be reported to dmesg, so that either old or
+		  malicious userspace programs can be identified.
+
+endchoice
+
 config CMDLINE_BOOL
 	bool "Built-in kernel command line"
 	---help---

arch/x86/entry/vsyscall/vsyscall_64.c

@@ -38,7 +38,14 @@
 #define CREATE_TRACE_POINTS
 #include "vsyscall_trace.h"
 
-static enum { EMULATE, NATIVE, NONE } vsyscall_mode = EMULATE;
+static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
+#ifdef CONFIG_LEGACY_VSYSCALL_NATIVE
+	NATIVE;
+#elif CONFIG_LEGACY_VSYSCALL_NONE
+	NONE;
+#else
+	EMULATE;
+#endif
 
 static int __init vsyscall_setup(char *str)
 {