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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id d5sm4039400pju.28.2021.08.17.23.48.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Aug 2021 23:48:41 -0700 (PDT) Date: Tue, 17 Aug 2021 23:48:40 -0700 From: Kees Cook To: Vincent MAILHOL Cc: Wolfgang Grandegger , Marc Kleine-Budde , "David S. Miller" , Jakub Kicinski , Arunachalam Santhanam , linux-can , netdev , open list , linux-hardening@vger.kernel.org Subject: Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array Message-ID: <202108172320.1540EC10C@keescook> References: <20210818034010.800652-1-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-can@vger.kernel.org On Wed, Aug 18, 2021 at 02:13:51PM +0900, Vincent MAILHOL wrote: > On Wed. 18 Aug 2021 at 12:40, Kees Cook wrote: > > While raw_msg isn't a fixed size, it does have a maximum size. Adjust the > > struct to represent this and avoid the following warning when building > > with -Wzero-length-bounds: > > > > drivers/net/can/usb/etas_es58x/es58x_fd.c: In function 'es58x_fd_tx_can_msg': > > drivers/net/can/usb/etas_es58x/es58x_fd.c:360:35: warning: array subscript 65535 is outside the bounds of an interior zero-length array 'u8[0]' {aka 'unsigned char[]'} [-Wzero-length-bounds] > > 360 | tx_can_msg = (typeof(tx_can_msg))&es58x_fd_urb_cmd->raw_msg[msg_len]; > > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > In file included from drivers/net/can/usb/etas_es58x/es58x_core.h:22, > > from drivers/net/can/usb/etas_es58x/es58x_fd.c:17: > > drivers/net/can/usb/etas_es58x/es58x_fd.h:231:6: note: while referencing 'raw_msg' > > 231 | u8 raw_msg[0]; > > | ^~~~~~~ > > > > Cc: Wolfgang Grandegger > > Cc: Marc Kleine-Budde > > Cc: "David S. Miller" > > Cc: Jakub Kicinski > > Cc: Arunachalam Santhanam > > Cc: Vincent Mailhol > > Cc: linux-can@vger.kernel.org > > Cc: netdev@vger.kernel.org > > Signed-off-by: Kees Cook > > --- > > drivers/net/can/usb/etas_es58x/es581_4.h | 2 +- > > drivers/net/can/usb/etas_es58x/es58x_fd.h | 2 +- > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/drivers/net/can/usb/etas_es58x/es581_4.h b/drivers/net/can/usb/etas_es58x/es581_4.h > > index 4bc60a6df697..af38c4938859 100644 > > --- a/drivers/net/can/usb/etas_es58x/es581_4.h > > +++ b/drivers/net/can/usb/etas_es58x/es581_4.h > > @@ -192,7 +192,7 @@ struct es581_4_urb_cmd { > > struct es581_4_rx_cmd_ret rx_cmd_ret; > > __le64 timestamp; > > u8 rx_cmd_ret_u8; > > - u8 raw_msg[0]; > > + u8 raw_msg[USHRT_MAX]; > > } __packed; > > > > __le16 reserved_for_crc16_do_not_use; > > diff --git a/drivers/net/can/usb/etas_es58x/es58x_fd.h b/drivers/net/can/usb/etas_es58x/es58x_fd.h > > index ee18a87e40c0..e0319b8358ef 100644 > > --- a/drivers/net/can/usb/etas_es58x/es58x_fd.h > > +++ b/drivers/net/can/usb/etas_es58x/es58x_fd.h > > @@ -228,7 +228,7 @@ struct es58x_fd_urb_cmd { > > struct es58x_fd_tx_ack_msg tx_ack_msg; > > __le64 timestamp; > > __le32 rx_cmd_ret_le32; > > - u8 raw_msg[0]; > > + u8 raw_msg[USHRT_MAX]; > > } __packed; > > > > __le16 reserved_for_crc16_do_not_use; > > -- > > 2.30.2 > > raw_msg is part of a union so its maximum size is implicitly the > biggest size of the other member of that union: Yup, understood. See below... > > | struct es58x_fd_urb_cmd { > | __le16 SOF; > | u8 cmd_type; > | u8 cmd_id; > | u8 channel_idx; > | __le16 msg_len; > | > | union { > | struct es58x_fd_tx_conf_msg tx_conf_msg; > | u8 tx_can_msg_buf[ES58X_FD_TX_BULK_MAX * ES58X_FD_CANFD_TX_LEN]; > | u8 rx_can_msg_buf[ES58X_FD_RX_BULK_MAX * ES58X_FD_CANFD_RX_LEN]; > | struct es58x_fd_echo_msg echo_msg[ES58X_FD_ECHO_BULK_MAX]; > | struct es58x_fd_rx_event_msg rx_event_msg; > | struct es58x_fd_tx_ack_msg tx_ack_msg; > | __le64 timestamp; > | __le32 rx_cmd_ret_le32; > | u8 raw_msg[0]; > | } __packed; > | > | __le16 reserved_for_crc16_do_not_use; > | } __packed; > > ram_msg can then be used to manipulate the other fields at the byte level. > I am sorry but I fail to understand why this is an issue. The issue is with using a 0-element array (these are being removed from the kernel[1] so we can add -Warray-bounds). Normally in this situation I would replace the 0-element array with a flexible array, but this case is unusual in several ways: - There is a trailing struct member (reserved_for_crc16_do_not_use), which is never accessed (good), and documented as "please never access this". - struct es58x_fd_urb_cmd is statically allocated (it is written into from the URB handler). - The message lengths coming from the USB device are stored in a u16, which looked like it was possible to overflow the buffer. In taking a closer look, I see that the URB command length is checked, and the in-data length is checked as well, so the overflow concern appears to be addressed. > Also, the proposed fix drastically increases the size of the structure. Indeed. I will send a v2, now that I see that the overflow concern isn't an issue. Thanks! -Kees [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays -- Kees Cook