x86/vsyscall: Remove CONFIG_LEGACY_VSYSCALL_EMULATE

CONFIG_LEGACY_VSYSCALL_EMULATE is, as far as I know, only needed for the
combined use of exotic and outdated debugging mechanisms with outdated
binaries. At this point, no one should be using it. Eventually, dynamic
switching of vsyscalls will be implemented, but this is much more
complicated to support in EMULATE mode than XONLY mode.

So let's force all the distros off of EMULATE mode. If anyone actually
needs it, they can set vsyscall=emulate, and the kernel can then get
away with refusing to support newer security models if that option is
set.

  [ bp: Remove "we"s. ]

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Florian Weimer <[email protected]>
Link: https://lore.kernel.org/r/898932fe61db6a9d61bc2458fa2f6049f1ca9f5c.1652290558.git.luto@kernel.org

Author: amluto

arch/x86/Kconfig

@@ -2326,28 +2326,16 @@ choice
 	  it can be used to assist security vulnerability exploitation.
 
 	  This setting can be changed at boot time via the kernel command
-	  line parameter vsyscall=[emulate|xonly|none].
+	  line parameter vsyscall=[emulate|xonly|none].  Emulate mode
+	  is deprecated and can only be enabled using the kernel command
+	  line.
 
 	  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 execution only".
 
-	config LEGACY_VSYSCALL_EMULATE
-		bool "Full emulation"
-		help
-		  The kernel traps and emulates calls into the fixed vsyscall
-		  address mapping. This makes the mapping non-executable, but
-		  it still contains readable known contents, which could be
-		  used in certain rare security vulnerability exploits. This
-		  configuration is recommended when using legacy userspace
-		  that still uses vsyscalls along with legacy binary
-		  instrumentation tools that require code to be readable.
-
-		  An example of this type of legacy userspace is running
-		  Pin on an old binary that still uses vsyscalls.
-
 	config LEGACY_VSYSCALL_XONLY
 		bool "Emulate execution only"
 		help

arch/x86/entry/vsyscall/vsyscall_64.c

@@ -48,7 +48,7 @@ static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
 #elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
 	XONLY;
 #else
-	EMULATE;
+	#error VSYSCALL config is broken
 #endif
 
 static int __init vsyscall_setup(char *str)