lib: Add a generic version of devmem_is_allowed()

As part of adding support for STRICT_DEVMEM to the RISC-V port, Zong
provided a devmem_is_allowed() implementation that's exactly the same as
all the others I checked.  Instead I'm adding a generic version, which
will soon be used.

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

lib/devmem_is_allowed.c

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * A generic version of devmem_is_allowed.
+ *
+ * Based on arch/arm64/mm/mmap.c
+ *
+ * Copyright (C) 2020 Google, Inc.
+ * Copyright (C) 2012 ARM Ltd.
+ */
+
+#include <linux/mm.h>
+#include <linux/ioport.h>
+
+/*
+ * devmem_is_allowed() checks to see if /dev/mem access to a certain address
+ * is valid. The argument is a physical page number.  We mimic x86 here by
+ * disallowing access to system RAM as well as device-exclusive MMIO regions.
+ * This effectively disable read()/write() on /dev/mem.
+ */
+int devmem_is_allowed(unsigned long pfn)
+{
+	if (iomem_is_exclusive(pfn << PAGE_SHIFT))
+		return 0;
+	if (!page_is_ram(pfn))
+		return 1;
+	return 0;
+}