firmware: coreboot: Check size of table entry and split memcpy

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]/ open here

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]>

---
v2: move flex array to struct coreboot_device (julius)
v1: https://lore.kernel.org/lkml/[email protected]

---
 drivers/firmware/google/coreboot_table.c | 9 +++++++--
 drivers/firmware/google/coreboot_table.h | 1 +
 2 files changed, 8 insertions(+), 2 deletions(-)

drivers/firmware/google/coreboot_table.c +7 -2

index 2652c396c423..564a3c908838 100644
@@ -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, entry, entry->size);
 
                switch (device->entry.tag) {
                case LB_TAG_CBMEM_ENTRY:

drivers/firmware/google/coreboot_table.h +1 -1

index 37f4d335a606..d814dca33a08 100644
@@ -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);
        };
 };
 
--

2.34.1

Reviewed-by Julius Werner <[email protected]>

drivers/firmware/google/coreboot_table.c static int coreboot_table_populate(struct device *dev, void *ptr) · line 108

Quoted 2 lines

drivers/firmware/google/coreboot_table.c +1 -1

-               memcpy(&device->entry, ptr_entry, entry->size);
+               memcpy(device->raw, entry, entry->size);

nit: It's a bit odd to change the source pointer from ptr_entry to entry here. Technically the static analyzer would be within its rights to give you a warning for that as well, because you're now "overrunning" the source struct instead of the destination one.