[PATCH] firmware: coreboot: Check size of table entry and split memcpy

[Kees Cook <keescook@chromium.org>]

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_table_entry, and 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 <pmenzel@molgen.mpg.de>
Link: https://lore.kernel.org/all/03ae2704-8c30-f9f0-215b-7cdf4ad35a9a@molgen.mpg.de/
Cc: Jack Rosenthal <jrosenth@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Julius Werner <jwerner@chromium.org>
Cc: Brian Norris <briannorris@chromium.org>
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/firmware/google/coreboot_table.c | 8 +++++++-
 drivers/firmware/google/coreboot_table.h | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/google/coreboot_table.c b/drivers/firmware/google/coreboot_table.c
index 2652c396c423..102edfb4eb7b 100644
--- a/drivers/firmware/google/coreboot_table.c
+++ b/drivers/firmware/google/coreboot_table.c
@@ -93,6 +93,11 @@ static int coreboot_table_populate(struct device *dev, void *ptr)
 	for (i = 0; i < header->table_entries; i++) {
 		entry = ptr_entry;
 
+		if (entry->size < sizeof(*entry)) {
+			dev_warn(dev, "coreboot table entry too small!\n");
+			return -EINVAL;
+		}
+
 		device = kzalloc(sizeof(struct device) + entry->size, GFP_KERNEL);
 		if (!device)
 			return -ENOMEM;
@@ -100,7 +105,8 @@ static int coreboot_table_populate(struct device *dev, void *ptr)
 		device->dev.parent = dev;
 		device->dev.bus = &coreboot_bus_type;
 		device->dev.release = coreboot_device_release;
-		memcpy(&device->entry, ptr_entry, entry->size);
+		device->entry = *entry;
+		memcpy(device->entry.data, entry->data, entry->size - sizeof(*entry));
 
 		switch (device->entry.tag) {
 		case LB_TAG_CBMEM_ENTRY:
diff --git a/drivers/firmware/google/coreboot_table.h b/drivers/firmware/google/coreboot_table.h
index 37f4d335a606..2a2cea79204b 100644
--- a/drivers/firmware/google/coreboot_table.h
+++ b/drivers/firmware/google/coreboot_table.h
@@ -29,6 +29,7 @@ struct coreboot_table_header {
 struct coreboot_table_entry {
 	u32 tag;
 	u32 size;
+	u8 data[];	/* Size here is: "size - (sizeof(u32) * 2)" */
 };
 
 /* Points to a CBMEM entry */
-- 
2.34.1