Re: [PATCH] media: cec: extron: validate response prefixes

[Hans Verkuil <hverkuil+cisco@kernel.org>]

Hi Pengpeng Hou,

On 30/08/2026 15:03, Pengpeng Hou wrote:
> extron_interrupt() dispatches completed serial lines by comparing fixed
> prefixes and reading fixed offsets without first checking the current line
> length. Short lines can therefore consume stale bytes beyond the
> NUL-terminated message item.
> 
> Require each response type to contain the complete dispatch prefix before
> reading its fixed fields.
> 
> Fixes: 056f2821b631 ("media: cec: extron-da-hd-4k-plus: add the Extron DA HD 4K Plus CEC driver")
> Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
> ---
>  .../usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c    | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c b/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
> index 3c6ce6f3d93e7..66780b3cf9188 100644
> --- a/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
> +++ b/drivers/media/cec/usb/extron-da-hd-4k-plus/extron-da-hd-4k-plus.c
> @@ -851,13 +851,13 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
>  		if (debug)
>  			dev_info(extron->dev, "received %s\n", extron->data);
>  		extron->idx = 0;
> -		if (!memcmp(extron->data, "Sig", 3) &&
> +		if (extron->len >= 5 && !memcmp(extron->data, "Sig", 3) &&

All these extra checks makes the code fragile (i.e. easy to make mistakes), esp. if new
prefix checks are added in the future.

How about just replace:

                extron->data[extron->len] = 0;

with:

                /*
                 * Prevent the following prefix tests from using stale data
                 * from beyond extron->len
                 */
                memset(extron->data + extron->len, 0,
                       min(10, sizeof(extron->data) - extron->len));

This ensures that bytes extron->len to extron->len + 9 are all set to 0 and you won't hit
stale data.

Regards,

	Hans

>  		    extron->data[4] == '*') {
>  			extron_process_signal_change(extron, extron->data + 3);
> -		} else if (!memcmp(extron->data, "Hdcp", 4) &&
> +		} else if (extron->len >= 6 && !memcmp(extron->data, "Hdcp", 4) &&
>  			   extron->data[5] == '*') {
>  			extron_process_edid_change(extron, extron->data + 4);
> -		} else if (!memcmp(extron->data, "DcecI", 5) &&
> +		} else if (extron->len >= 6 && !memcmp(extron->data, "DcecI", 5) &&
>  			   extron->data[5] >= '1' &&
>  			   extron->data[5] < '1' + extron->num_in_ports) {
>  			unsigned int p = extron->data[5] - '1';
> @@ -865,7 +865,7 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
>  			p += extron->num_out_ports;
>  			extron_process_tx_done(extron->ports[p],
>  					       extron->data[extron->len - 1]);
> -		} else if (!memcmp(extron->data, "Ceci", 4) &&
> +		} else if (extron->len >= 6 && !memcmp(extron->data, "Ceci", 4) &&
>  			   extron->data[4] >= '1' &&
>  			   extron->data[4] < '1' + extron->num_in_ports &&
>  			   extron->data[5] == '*') {
> @@ -874,14 +874,14 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
>  			p += extron->num_out_ports;
>  			extron_process_received(extron->ports[p],
>  						extron->data + 6);
> -		} else if (!memcmp(extron->data, "DcecO", 5) &&
> +		} else if (extron->len >= 6 && !memcmp(extron->data, "DcecO", 5) &&
>  			   extron->data[5] >= '1' &&
>  			   extron->data[5] < '1' + extron->num_out_ports) {
>  			unsigned int p = extron->data[5] - '1';
>  
>  			extron_process_tx_done(extron->ports[p],
>  					       extron->data[extron->len - 1]);
> -		} else if (!memcmp(extron->data, "Ceco", 4) &&
> +		} else if (extron->len >= 6 && !memcmp(extron->data, "Ceco", 4) &&
>  			   extron->data[4] >= '1' &&
>  			   extron->data[4] < '1' + extron->num_out_ports &&
>  			   extron->data[5] == '*') {
> @@ -889,7 +889,7 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
>  
>  			extron_process_received(extron->ports[p],
>  						extron->data + 6);
> -		} else if (!memcmp(extron->data, "Pceco", 5) &&
> +		} else if (extron->len >= 7 && !memcmp(extron->data, "Pceco", 5) &&
>  			   extron->data[5] >= '1' &&
>  			   extron->data[5] < '1' + extron->num_out_ports) {
>  			unsigned int p = extron->data[5] - '1';
> @@ -899,7 +899,7 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
>  				   &tmp_pa[0], &tmp_pa[1]) == 2)
>  				extron_phys_addr_change(extron->ports[p],
>  							tmp_pa[0] << 8 | tmp_pa[1]);
> -		} else if (!memcmp(extron->data, "Pceci", 5) &&
> +		} else if (extron->len >= 7 && !memcmp(extron->data, "Pceci", 5) &&
>  			   extron->data[5] >= '1' &&
>  			   extron->data[5] < '1' + extron->num_in_ports) {
>  			unsigned int p = extron->data[5] - '1';
> @@ -910,7 +910,7 @@ static irqreturn_t extron_interrupt(struct serio *serio, unsigned char data,
>  				   &tmp_pa[0], &tmp_pa[1]) == 2)
>  				extron_phys_addr_change(extron->ports[p],
>  							tmp_pa[0] << 8 | tmp_pa[1]);
> -		} else if (!memcmp(extron->data, "EdidR", 5) &&
> +		} else if (extron->len >= 7 && !memcmp(extron->data, "EdidR", 5) &&
>  			   extron->data[5] >= '1' &&
>  			   extron->data[5] < '1' + extron->num_ports &&
>  			   extron->data[6] == '*') {
> 
> base-commit: 08dbfad3f5040f5bdb6c529da20d6d4e81fefd72