linux-kernel - Re: [PATCH] comedi: fix divide-by-zero in comedi_buf

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Message-ID: <68d34db4.a70a0220.1b52b.02bc.GAE@google.com>
Date: Tue, 23 Sep 2025 18:47:32 -0700
From: syzbot <syzbot+f6c3c066162d2c43a66c@...kaller.appspotmail.com>
To: kartikey406@...il.com
Cc: kartikey406@...il.com, stable@...r.kernel.org, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] comedi: fix divide-by-zero in comedi_buf_munge()

> #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

This crash does not have a reproducer. I cannot test it.

>
> 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.
>
> 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@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=f6c3c066162d2c43a66c
> Cc: stable@...r.kernel.org
> Signed-off-by: Deepanshu Kartikey <kartikey406@...il.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;
> +	}
>  
>  	/* don't munge partial samples */
>  	num_bytes -= num_bytes % num_sample_bytes;
> -- 
> 2.43.0
>