asm-generic: uaccess: add missing access_ok() check to strnlen_user()

The strnlen_user() function was missing a access_ok() check on the pointer
given.  We've had cases on Blackfin systems where test programs caused
kernel crashes here because userspace passed up a NULL/-1 pointer and the
kernel gladly attempted to run strlen() on it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/uaccess.h

@@ -291,6 +291,8 @@ strncpy_from_user(char *dst, const char __user *src, long count)
 #ifndef strnlen_user
 static inline long strnlen_user(const char __user *src, long n)
 {
+	if (!access_ok(VERIFY_READ, src, 1))
+		return 0;
 	return strlen((void * __force)src) + 1;
 }
 #endif