firmware: coreboot: Check size of table entry and use flex-array

The memcpy() of the data following a coreboot_table_entry couldn't
be evaluated by the compiler under CONFIG_FORTIFY_SOURCE. To make it
easier to reason about, add an explicit flexible array member to struct
coreboot_device so the entire entry can be copied at once. Additionally,
validate the sizes before copying. Avoids this run-time false positive
warning:

  memcpy: detected field-spanning write (size 168) of single field "&device->entry" at drivers/firmware/google/coreboot_table.c:103 (size 8)

Reported-by: Paul Menzel <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Cc: Jack Rosenthal <[email protected]>
Cc: Guenter Roeck <[email protected]>
Cc: Julius Werner <[email protected]>
Cc: Brian Norris <[email protected]>
Cc: Stephen Boyd <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Julius Werner <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Stephen Boyd <[email protected]>
Reviewed-by: Jack Rosenthal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

drivers/firmware/google/coreboot_table.c

@@ -93,14 +93,19 @@ static int coreboot_table_populate(struct device *dev, void *ptr)
 	for (i = 0; i < header->table_entries; i++) {
 		entry = ptr_entry;
 
-		device = kzalloc(sizeof(struct device) + entry->size, GFP_KERNEL);
+		if (entry->size < sizeof(*entry)) {
+			dev_warn(dev, "coreboot table entry too small!\n");
+			return -EINVAL;
+		}
+
+		device = kzalloc(sizeof(device->dev) + entry->size, GFP_KERNEL);
 		if (!device)
 			return -ENOMEM;
 
 		device->dev.parent = dev;
 		device->dev.bus = &coreboot_bus_type;
 		device->dev.release = coreboot_device_release;
-		memcpy(&device->entry, ptr_entry, entry->size);
+		memcpy(device->raw, ptr_entry, entry->size);
 
 		switch (device->entry.tag) {
 		case LB_TAG_CBMEM_ENTRY:

drivers/firmware/google/coreboot_table.h

@@ -79,6 +79,7 @@ struct coreboot_device {
 		struct lb_cbmem_ref cbmem_ref;
 		struct lb_cbmem_entry cbmem_entry;
 		struct lb_framebuffer framebuffer;
+		DECLARE_FLEX_ARRAY(u8, raw);
 	};
 };