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

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

[Kees Cook]

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

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

[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.

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

[Kees Cook]

On Fri, Jan 06, 2023 at 04:03:35PM +0100, Julius Werner wrote:
> 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?


diff --git a/drivers/firmware/google/coreboot_table.c b/drivers/firmware/google/coreboot_table.c
index 2652c396c423..564a3c908838 100644
--- a/drivers/firmware/google/coreboot_table.c
+++ b/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, entry, entry->size);
 
 		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..d814dca33a08 100644
--- a/drivers/firmware/google/coreboot_table.h
+++ b/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);
 	};
 };
 

-- 
Kees Cook

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

[Julius Werner]

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.)

On Fri, Jan 6, 2023 at 9:35 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Jan 06, 2023 at 04:03:35PM +0100, Julius Werner wrote:
> > 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?
>
>
> diff --git a/drivers/firmware/google/coreboot_table.c b/drivers/firmware/google/coreboot_table.c
> index 2652c396c423..564a3c908838 100644
> --- a/drivers/firmware/google/coreboot_table.c
> +++ b/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, entry, entry->size);
>
>                 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..d814dca33a08 100644
> --- a/drivers/firmware/google/coreboot_table.h
> +++ b/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);
>         };
>  };
>
>
> --
> Kees Cook