of: Add memory limiting function for flattened devicetrees

Buggy bootloaders may pass bogus memory entries in the devicetree.
Add of_fdt_limit_memory to add an upper bound on the number of
entries that can be present in the devicetree.
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

drivers/of/fdt.c

@@ -26,6 +26,54 @@
 #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
 #include <asm/page.h>
 
+/*
+ * of_fdt_limit_memory - limit the number of regions in the /memory node
+ * @limit: maximum entries
+ *
+ * Adjust the flattened device tree to have at most 'limit' number of
+ * memory entries in the /memory node. This function may be called
+ * any time after initial_boot_param is set.
+ */
+void of_fdt_limit_memory(int limit)
+{
+	int memory;
+	int len;
+	const void *val;
+	int nr_address_cells = OF_ROOT_NODE_ADDR_CELLS_DEFAULT;
+	int nr_size_cells = OF_ROOT_NODE_SIZE_CELLS_DEFAULT;
+	const uint32_t *addr_prop;
+	const uint32_t *size_prop;
+	int root_offset;
+	int cell_size;
+
+	root_offset = fdt_path_offset(initial_boot_params, "/");
+	if (root_offset < 0)
+		return;
+
+	addr_prop = fdt_getprop(initial_boot_params, root_offset,
+				"#address-cells", NULL);
+	if (addr_prop)
+		nr_address_cells = fdt32_to_cpu(*addr_prop);
+
+	size_prop = fdt_getprop(initial_boot_params, root_offset,
+				"#size-cells", NULL);
+	if (size_prop)
+		nr_size_cells = fdt32_to_cpu(*size_prop);
+
+	cell_size = sizeof(uint32_t)*(nr_address_cells + nr_size_cells);
+
+	memory = fdt_path_offset(initial_boot_params, "/memory");
+	if (memory > 0) {
+		val = fdt_getprop(initial_boot_params, memory, "reg", &len);
+		if (len > limit*cell_size) {
+			len = limit*cell_size;
+			pr_debug("Limiting number of entries to %d\n", limit);
+			fdt_setprop(initial_boot_params, memory, "reg", val,
+					len);
+		}
+	}
+}
+
 /**
  * of_fdt_is_compatible - Return true if given node from the given blob has
  * compat in its compatible list

include/linux/of_fdt.h

@@ -86,6 +86,7 @@ extern void unflatten_and_copy_device_tree(void);
 extern void early_init_devtree(void *);
 extern void early_get_first_memblock_info(void *, phys_addr_t *);
 extern u64 fdt_translate_address(const void *blob, int node_offset);
+extern void of_fdt_limit_memory(int limit);
 #else /* CONFIG_OF_FLATTREE */
 static inline void early_init_fdt_scan_reserved_mem(void) {}
 static inline const char *of_flat_dt_get_machine_name(void) { return NULL; }