vsyscall: Fix permissions for emulate mode with KAISER/PTI

The backport of KAISER to 4.4 turned vsyscall emulate mode into native
mode.  Add a vsyscall_pgprot variable to hold the correct page
protections, like Borislav and Hugh did for 3.2 and 3.18.

Cc: Borislav Petkov <bp@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/entry/vsyscall/vsyscall_64.c

@@ -46,6 +46,7 @@ static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
 #else
 	EMULATE;
 #endif
+unsigned long vsyscall_pgprot = __PAGE_KERNEL_VSYSCALL;
 
 static int __init vsyscall_setup(char *str)
 {
@@ -336,11 +337,11 @@ void __init map_vsyscall(void)
 	extern char __vsyscall_page;
 	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 
+	if (vsyscall_mode != NATIVE)
+		vsyscall_pgprot = __PAGE_KERNEL_VVAR;
 	if (vsyscall_mode != NONE)
 		__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
-			     vsyscall_mode == NATIVE
-			     ? PAGE_KERNEL_VSYSCALL
-			     : PAGE_KERNEL_VVAR);
+			     __pgprot(vsyscall_pgprot));
 
 	BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 		     (unsigned long)VSYSCALL_ADDR);

arch/x86/include/asm/vsyscall.h

@@ -13,6 +13,7 @@ extern void map_vsyscall(void);
  */
 extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);
 extern bool vsyscall_enabled(void);
+extern unsigned long vsyscall_pgprot;
 #else
 static inline void map_vsyscall(void) {}
 static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)

arch/x86/mm/kaiser.c

@@ -345,7 +345,7 @@ void __init kaiser_init(void)
 	if (vsyscall_enabled())
 		kaiser_add_user_map_early((void *)VSYSCALL_ADDR,
 					  PAGE_SIZE,
-					   __PAGE_KERNEL_VSYSCALL);
+					  vsyscall_pgprot);
 
 	for_each_possible_cpu(cpu) {
 		void *percpu_vaddr = __per_cpu_user_mapped_start +