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

PATCH Superseded Kees Cook 2 participants 4 messages Jan 6, 2023

v1 of 3 Superseded Latest is v3 →

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Kees Cook

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#0 lore raw

PATCH 1/1 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_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 <[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]>
---
 drivers/firmware/google/coreboot_table.c | 8 +++++++-
 drivers/firmware/google/coreboot_table.h | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)
drivers/firmware/google/coreboot_table.c +7 -1
index 2652c396c423..102edfb4eb7b 100644
@@ -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:
drivers/firmware/google/coreboot_table.h +1 -0
index 37f4d335a606..2a2cea79204b 100644
@@ -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 */
Signature · 1 line
2.34.1
#1 lore raw

Have you considered adding the flexible array member directly to the union in struct coreboot_device instead? I think that would make this a bit simpler by not having to copy header and data portion separately.

author
#2 lore raw
Quoted 4 lines · Julius Werner

Have you considered adding the flexible array member directly to the union in struct coreboot_device instead? I think that would make this a bit simpler by not having to copy header and data portion separately.

Are you thinking something like this?

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 -0
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);
 	};
 };
Signature · 1 line
Kees Cook
#3 lore raw

Yeah, exactly. Isn't that a bit simpler? (I wasn't aware of the flexible array member in union issue, but it sounds like we have a neat solution for that already.)

Quoted 51 lines · Kees Cook
Quoted 4 lines · Julius Werner

Have you considered adding the flexible array member directly to the union in struct coreboot_device instead? I think that would make this a bit simpler by not having to copy header and data portion separately.

Are you thinking something like this?

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);
        };
 };
 
 
--

Kees Cook