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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <3f7b60e8-9df8-414a-afd2-2caca7d43209@mev.co.uk>
Date: Wed, 24 Sep 2025 10:03:54 +0100
From: Ian Abbott <abbotti@....co.uk>
To: Deepanshu Kartikey <kartikey406@...il.com>,
 H Hartley Sweeten <hsweeten@...ionengravers.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 syzbot+f6c3c066162d2c43a66c@...kaller.appspotmail.com
Subject: Re: [PATCH] comedi: fix divide-by-zero in comedi_buf_munge()

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@...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;
> +	}

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;


-- 
-=( Ian Abbott <abbotti@....co.uk> || MEV Ltd. is a company  )=-
-=( registered in England & Wales.  Regd. number: 02862268.  )=-
-=( Regd. addr.: S11 & 12 Building 67, Europa Business Park, )=-
-=( Bird Hall Lane, STOCKPORT, SK3 0XA, UK. || www.mev.co.uk )=-