Re: [PATCH] comedi: fix divide-by-zero in comedi_buf_munge()

[Ian Abbott <abbotti@mev.co.uk>]

On 24/09/2025 02:56, Deepanshu Kartikey wrote:
> The comedi_buf_munge() function performs a modulo operation
> `async->munge_chan %= async->cmd.chanlist_len` without first
> checking if chanlist_len is zero. If a user program submits
> a command with chanlist_len set to zero, this causes a
> divide-by-zero error when the device processes data in the
> interrupt handler path.

I don't think it should happen for supported hardware devices that are 
working properly, but it helps to defend against the unexpected.

> Add a check for zero chanlist_len at the beginning of the
> function, similar to the existing checks for !map and
> CMDF_RAWDATA flag. When chanlist_len is zero, update
> munge_count and return early, indicating the data was
> handled without munging.
> 
> This prevents potential kernel panics from malformed user commands.
> 
> Reported-by: syzbot+f6c3c066162d2c43a66c@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=f6c3c066162d2c43a66c
> Cc: stable@vger.kernel.org
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
>   drivers/comedi/comedi_buf.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/comedi/comedi_buf.c b/drivers/comedi/comedi_buf.c
> index 002c0e76baff..786f888299ce 100644
> --- a/drivers/comedi/comedi_buf.c
> +++ b/drivers/comedi/comedi_buf.c
> @@ -321,6 +321,11 @@ static unsigned int comedi_buf_munge(struct comedi_subdevice *s,
>   		async->munge_count += num_bytes;
>   		return num_bytes;
>   	}
> +
> +	if (async->cmd.chanlist_len == 0) {
> +		async->munge_count += num_bytes;
> +		return num_bytes;
> +	}

This `if` statement could be merged with the preceding `if` statement 
that has the same body, which would save a few lines.

>   
>   	/* don't munge partial samples */
>   	num_bytes -= num_bytes % num_sample_bytes;


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