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

From: Ian Abbott
Date: Wed Mar 13 2019 - 14:57:23 EST

On 04/03/2019 14:33, 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:!topic/comedi_list/4t9WlHzMhKM

Fixes: f164cbf98fa8 ("staging: comedi: ni_mio_common: add finite regeneration to dio output")

If it's not too late, it would be nice if the following "Reported-by:" and "Tested-by:" lines could be added (or I can resend with these lines included if necessary). It's no big deal if this is too late. I'll live with it. Thanks.

Reported-by: Ivan Vasilyev <grabesstimme@xxxxxxxxx>
Tested-by: Ivan Vasilyev <grabesstimme@xxxxxxxxx>

Cc: <stable@xxxxxxxxxxxxxxx> # 4.6+
Cc: Spencer E. Olson <olsonse@xxxxxxxxx>
Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
drivers/staging/comedi/comedidev.h | 2 ++
drivers/staging/comedi/drivers.c | 33 ++++++++++++++++---
.../staging/comedi/drivers/ni_mio_common.c | 10 ++++--
3 files changed, 38 insertions(+), 7 deletions(-)

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