x86/mm: Add vsyscall address helper

We will shortly be using this check in two locations.  Put it in
a helper before we do so.

Let's also insert PAGE_MASK instead of the open-coded ~0xfff.
It is easier to read and also more obviously correct considering
the implicit type conversion that has to happen when it is not
an implicit 'unsigned long'.

Cc: x86@kernel.org
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180928160228.C593509B@viggo.jf.intel.com

arch/x86/mm/fault.c

@@ -840,6 +840,15 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 	show_opcodes(regs, loglvl);
 }
 
+/*
+ * The (legacy) vsyscall page is the long page in the kernel portion
+ * of the address space that has user-accessible permissions.
+ */
+static bool is_vsyscall_vaddr(unsigned long vaddr)
+{
+	return (vaddr & PAGE_MASK) == VSYSCALL_ADDR;
+}
+
 static void
 __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 		       unsigned long address, u32 *pkey, int si_code)
@@ -869,7 +878,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 		 * emulation.
 		 */
 		if (unlikely((error_code & X86_PF_INSTR) &&
-			     ((address & ~0xfff) == VSYSCALL_ADDR))) {
+			     is_vsyscall_vaddr(address))) {
 			if (emulate_vsyscall(regs, address))
 				return;
 		}