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Date: Mon, 4 Mar 2024 16:45:11 -0600
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Subject: Re: [PATCH v2] greybus: Avoid fake flexible array for response data
Content-Language: en-US
To: Kees Cook <keescook@chromium.org>, Alex Elder <elder@kernel.org>
Cc: Viresh Kumar <vireshk@kernel.org>, Johan Hovold <johan@kernel.org>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 "Gustavo A . R . Silva" <gustavo@embeddedor.com>,
 greybus-dev@lists.linaro.org, linux-staging@lists.linux.dev,
 "Gustavo A. R. Silva" <gustavoars@kernel.org>, linux-kernel@vger.kernel.org,
 linux-hardening@vger.kernel.org
References: <20240304211940.it.083-kees@kernel.org>
From: Alex Elder <elder@ieee.org>
In-Reply-To: <20240304211940.it.083-kees@kernel.org>
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On 3/4/24 3:19 PM, Kees Cook wrote:
> FORTIFY_SOURCE has been ignoring 0-sized destinations while the kernel
> code base has been converted to flexible arrays. In order to enforce
> the 0-sized destinations (e.g. with __counted_by), the remaining 0-sized
> destinations need to be handled. Instead of converting an empty struct
> into using a flexible array, just directly use a pointer without any
> additional indirection. Remove struct gb_bootrom_get_firmware_response
> and struct gb_fw_download_fetch_firmware_response.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>

Thanks for adding the comments!  This looks good to me.

Reviewed-by: Alex Elder <elder@linaro.org>


I want to call attention to a few other spots that should
get a little more attention--related directly to what you're
doing here.

I noticed that the GB_CONTROL_TYPE_GET_MANIFEST response
structure also contains only a flexible array.  It might
be good to add a similar comment in gb_interface_enable(),
above this line:
         manifest = kmalloc(size, GFP_KERNEL);
The definition of the gb_control_get_manifest_response structure
could probably be replaced with a comment.


The response buffer for an I2C transfer consists only of incoming
data.  There is already a comment in gb_i2c_operation_create()
that says this:
         /* Response consists only of incoming data */
The definition of the gb_i2c_transfer_response structure should
then go away, in favor of a comment saying this.

The response buffer for a SPI transfer consists only of incoming
data.  It is used three times in "driver/staging/greybus/spilib.c":
- calc_rx_xfer_size() subtracts the size of the response structure,
   and that should be replaced by a comment (and the structure
   definition should go away)
- gb_spi_decode_response() takes the response structure as an
   argument.  That could be replaced with a void pointer instead,
   with a comment.
- gb_spi_transfer_one_message() is what passes the response buffer
   to gb_spi_decode_response(), and could be adjusted to reflect
   that the response consists only of data--rather than a struct
   containing only a flexible array.


Kees:  I'm *not* asking you to deal with these, I'm just mentioning
them to you.  My comments above (without someone else confirming)
are not sufficient to dictate how to address these.

					-Alex


> ---
> Cc: Alex Elder <elder@kernel.org>
> Cc: Viresh Kumar <vireshk@kernel.org>
> Cc: Johan Hovold <johan@kernel.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
> Cc: greybus-dev@lists.linaro.org
> Cc: linux-staging@lists.linux.dev
>   v2: add comments about removed structs
>   v1: https://patchwork.kernel.org/project/linux-hardening/patch/20240216232824.work.862-kees@kernel.org/
> ---
>   drivers/staging/greybus/bootrom.c         | 8 ++++----
>   drivers/staging/greybus/fw-download.c     | 8 ++++----
>   include/linux/greybus/greybus_protocols.h | 8 ++------
>   3 files changed, 10 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/staging/greybus/bootrom.c b/drivers/staging/greybus/bootrom.c
> index 79581457c4af..c0d338db6b52 100644
> --- a/drivers/staging/greybus/bootrom.c
> +++ b/drivers/staging/greybus/bootrom.c
> @@ -243,10 +243,10 @@ static int gb_bootrom_get_firmware(struct gb_operation *op)
>   	struct gb_bootrom *bootrom = gb_connection_get_data(op->connection);
>   	const struct firmware *fw;
>   	struct gb_bootrom_get_firmware_request *firmware_request;
> -	struct gb_bootrom_get_firmware_response *firmware_response;
>   	struct device *dev = &op->connection->bundle->dev;
>   	unsigned int offset, size;
>   	enum next_request_type next_request;
> +	u8 *firmware_response;
>   	int ret = 0;
>   
>   	/* Disable timeouts */
> @@ -280,15 +280,15 @@ static int gb_bootrom_get_firmware(struct gb_operation *op)
>   		goto unlock;
>   	}
>   
> -	if (!gb_operation_response_alloc(op, sizeof(*firmware_response) + size,
> -					 GFP_KERNEL)) {
> +	/* gb_bootrom_get_firmware_response contains only a byte array */
> +	if (!gb_operation_response_alloc(op, size, GFP_KERNEL)) {
>   		dev_err(dev, "%s: error allocating response\n", __func__);
>   		ret = -ENOMEM;
>   		goto unlock;
>   	}
>   
>   	firmware_response = op->response->payload;
> -	memcpy(firmware_response->data, fw->data + offset, size);
> +	memcpy(firmware_response, fw->data + offset, size);
>   
>   	dev_dbg(dev, "responding with firmware (offs = %u, size = %u)\n",
>   		offset, size);
> diff --git a/drivers/staging/greybus/fw-download.c b/drivers/staging/greybus/fw-download.c
> index 543692c567f9..a06065fb0c5e 100644
> --- a/drivers/staging/greybus/fw-download.c
> +++ b/drivers/staging/greybus/fw-download.c
> @@ -271,11 +271,11 @@ static int fw_download_fetch_firmware(struct gb_operation *op)
>   	struct gb_connection *connection = op->connection;
>   	struct fw_download *fw_download = gb_connection_get_data(connection);
>   	struct gb_fw_download_fetch_firmware_request *request;
> -	struct gb_fw_download_fetch_firmware_response *response;
>   	struct fw_request *fw_req;
>   	const struct firmware *fw;
>   	unsigned int offset, size;
>   	u8 firmware_id;
> +	u8 *response;
>   	int ret = 0;
>   
>   	if (op->request->payload_size != sizeof(*request)) {
> @@ -325,8 +325,8 @@ static int fw_download_fetch_firmware(struct gb_operation *op)
>   		goto put_fw;
>   	}
>   
> -	if (!gb_operation_response_alloc(op, sizeof(*response) + size,
> -					 GFP_KERNEL)) {
> +	/* gb_fw_download_fetch_firmware_response contains only a byte array */
> +	if (!gb_operation_response_alloc(op, size, GFP_KERNEL)) {
>   		dev_err(fw_download->parent,
>   			"error allocating fetch firmware response\n");
>   		ret = -ENOMEM;
> @@ -334,7 +334,7 @@ static int fw_download_fetch_firmware(struct gb_operation *op)
>   	}
>   
>   	response = op->response->payload;
> -	memcpy(response->data, fw->data + offset, size);
> +	memcpy(response, fw->data + offset, size);
>   
>   	dev_dbg(fw_download->parent,
>   		"responding with firmware (offs = %u, size = %u)\n", offset,
> diff --git a/include/linux/greybus/greybus_protocols.h b/include/linux/greybus/greybus_protocols.h
> index aeb8f9243545..820134b0105c 100644
> --- a/include/linux/greybus/greybus_protocols.h
> +++ b/include/linux/greybus/greybus_protocols.h
> @@ -232,9 +232,7 @@ struct gb_fw_download_fetch_firmware_request {
>   	__le32			size;
>   } __packed;
>   
> -struct gb_fw_download_fetch_firmware_response {
> -	__u8			data[0];
> -} __packed;
> +/* gb_fw_download_fetch_firmware_response contains no other data */
>   
>   /* firmware download release firmware request */
>   struct gb_fw_download_release_firmware_request {
> @@ -414,9 +412,7 @@ struct gb_bootrom_get_firmware_request {
>   	__le32			size;
>   } __packed;
>   
> -struct gb_bootrom_get_firmware_response {
> -	__u8			data[0];
> -} __packed;
> +/* gb_bootrom_get_firmware_response contains no other data */
>   
>   /* Bootrom protocol Ready to boot request */
>   struct gb_bootrom_ready_to_boot_request {