linux-kernel - Re: [PATCH] staging: comedi: ni_mio_common: Fix divide-by-zero for DIO cmdtest

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Message-ID: <20190305081629.GA13474@kadam>
Date:   Tue, 5 Mar 2019 14:10:31 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Ian Abbott <abbotti@....co.uk>
Cc:     devel@...verdev.osuosl.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        "Spencer E . Olson" <olsonse@...ch.edu>
Subject: Re: [PATCH] staging: comedi: ni_mio_common: Fix divide-by-zero for
 DIO cmdtest

On Mon, Mar 04, 2019 at 02:33:54PM +0000, Ian Abbott wrote:
> `ni_cdio_cmdtest()` validates Comedi asynchronous commands for the DIO
> subdevice (subdevice 2) of supported National Instruments M-series
> cards.  It is called when handling the `COMEDI_CMD` and `COMEDI_CMDTEST`
> ioctls for this subdevice.  There are two causes for a possible
> divide-by-zero error when validating that the `stop_arg` member of the
> passed-in command is not too large.
> 
> The first cause for the divide-by-zero is that calls to
> `comedi_bytes_per_scan()` are only valid once the command has been
> copied to `s->async->cmd`, but that copy is only done for the
> `COMEDI_CMD` ioctl.  For the `COMEDI_CMDTEST` ioctl, it will use
> whatever was left there by the previous `COMEDI_CMD` ioctl, if any.
> (This is very likely, as it is usual for the application to use
> `COMEDI_CMDTEST` before `COMEDI_CMD`.) If there has been no previous,
> valid `COMEDI_CMD` for this subdevice, then `comedi_bytes_per_scan()`
> will return 0, so the subsequent division in `ni_cdio_cmdtest()` of
> `s->async->prealloc_bufsz / comedi_bytes_per_scan(s)` will be a
> divide-by-zero error.  To fix this error, call a new function
> `comedi_bytes_per_scan_cmd(s, cmd)`, based on the existing
> `comedi_bytes_per_scan(s)` but using a specified `struct comedi_cmd` for
> its calculations.  (Also refactor `comedi_bytes_per_scan()` to call the
> new function.)
> 
> Once the first cause for the divide-by-zero has been fixed, the second
> cause is that `comedi_bytes_per_scan_cmd()` can legitimately return 0 if
> the `scan_end_arg` member of the `struct comedi_cmd` being tested is 0.
> Fix it by only performing the division (and validating that `stop_arg`
> is no more than the maximum value) if `comedi_bytes_per_scan_cmd()`
> returns a non-zero value.
> 
> The problem was reported on the COMEDI mailing list here:
> https://groups.google.com/forum/#!topic/comedi_list/4t9WlHzMhKM
> 

Can you give Ivan a Reported-by tag?  It's a public mailing list so
that shouldn't be a problem.

> Fixes: f164cbf98fa8 ("staging: comedi: ni_mio_common: add finite regeneration to dio output")
> Cc: <stable@...r.kernel.org> # 4.6+
> Cc: Spencer E. Olson <olsonse@...ch.edu>
> Signed-off-by: Ian Abbott <abbotti@....co.uk>
> ---

regards,
dan carpenter