m68k: fix access_ok for coldfire

While most m68k platforms use separate address spaces for user
and kernel space, at least coldfire does not, and the other
ones have a TASK_SIZE that is less than the entire 4GB address
range.

Using the default implementation of __access_ok() stops coldfire
user space from trivially accessing kernel memory.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/m68k/include/asm/uaccess.h

@@ -12,14 +12,17 @@
 #include <asm/extable.h>
 
 /* We let the MMU do all checking */
-static inline int access_ok(const void __user *addr,
+static inline int access_ok(const void __user *ptr,
 			    unsigned long size)
 {
-	/*
-	 * XXX: for !CONFIG_CPU_HAS_ADDRESS_SPACES this really needs to check
-	 * for TASK_SIZE!
-	 */
-	return 1;
+	unsigned long limit = TASK_SIZE;
+	unsigned long addr = (unsigned long)ptr;
+
+	if (IS_ENABLED(CONFIG_CPU_HAS_ADDRESS_SPACES) ||
+	    !IS_ENABLED(CONFIG_MMU))
+		return 1;
+
+	return (size <= limit) && (addr <= (limit - size));
 }
 
 /*