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Subject[PATCH 4.19 081/134] staging: comedi: ni_mio_common: Fix divide-by-zero for DIO cmdtest
Date
4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ian Abbott <abbotti@mev.co.uk>

commit bafd9c64056cd034a1174dcadb65cd3b294ff8f6 upstream.

`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

Reported-by: Ivan Vasilyev <grabesstimme@gmail.com>
Tested-by: Ivan Vasilyev <grabesstimme@gmail.com>
Fixes: f164cbf98fa8 ("staging: comedi: ni_mio_common: add finite regeneration to dio output")
Cc: <stable@vger.kernel.org> # 4.6+
Cc: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/staging/comedi/comedidev.h | 2 +
drivers/staging/comedi/drivers.c | 33 +++++++++++++++++++++----
drivers/staging/comedi/drivers/ni_mio_common.c | 10 +++++--
3 files changed, 38 insertions(+), 7 deletions(-)

--- a/drivers/staging/comedi/comedidev.h
+++ b/drivers/staging/comedi/comedidev.h
@@ -987,6 +987,8 @@ int comedi_dio_insn_config(struct comedi
unsigned int mask);
unsigned int comedi_dio_update_state(struct comedi_subdevice *s,
unsigned int *data);
+unsigned int comedi_bytes_per_scan_cmd(struct comedi_subdevice *s,
+ struct comedi_cmd *cmd);
unsigned int comedi_bytes_per_scan(struct comedi_subdevice *s);
unsigned int comedi_nscans_left(struct comedi_subdevice *s,
unsigned int nscans);
--- a/drivers/staging/comedi/drivers.c
+++ b/drivers/staging/comedi/drivers.c
@@ -381,11 +381,13 @@ unsigned int comedi_dio_update_state(str
EXPORT_SYMBOL_GPL(comedi_dio_update_state);

/**
- * comedi_bytes_per_scan() - Get length of asynchronous command "scan" in bytes
+ * comedi_bytes_per_scan_cmd() - Get length of asynchronous command "scan" in
+ * bytes
* @s: COMEDI subdevice.
+ * @cmd: COMEDI command.
*
* Determines the overall scan length according to the subdevice type and the
- * number of channels in the scan.
+ * number of channels in the scan for the specified command.
*
* For digital input, output or input/output subdevices, samples for
* multiple channels are assumed to be packed into one or more unsigned
@@ -395,9 +397,9 @@ EXPORT_SYMBOL_GPL(comedi_dio_update_stat
*
* Returns the overall scan length in bytes.
*/
-unsigned int comedi_bytes_per_scan(struct comedi_subdevice *s)
+unsigned int comedi_bytes_per_scan_cmd(struct comedi_subdevice *s,
+ struct comedi_cmd *cmd)
{
- struct comedi_cmd *cmd = &s->async->cmd;
unsigned int num_samples;
unsigned int bits_per_sample;

@@ -414,6 +416,29 @@ unsigned int comedi_bytes_per_scan(struc
}
return comedi_samples_to_bytes(s, num_samples);
}
+EXPORT_SYMBOL_GPL(comedi_bytes_per_scan_cmd);
+
+/**
+ * comedi_bytes_per_scan() - Get length of asynchronous command "scan" in bytes
+ * @s: COMEDI subdevice.
+ *
+ * Determines the overall scan length according to the subdevice type and the
+ * number of channels in the scan for the current command.
+ *
+ * For digital input, output or input/output subdevices, samples for
+ * multiple channels are assumed to be packed into one or more unsigned
+ * short or unsigned int values according to the subdevice's %SDF_LSAMPL
+ * flag. For other types of subdevice, samples are assumed to occupy a
+ * whole unsigned short or unsigned int according to the %SDF_LSAMPL flag.
+ *
+ * Returns the overall scan length in bytes.
+ */
+unsigned int comedi_bytes_per_scan(struct comedi_subdevice *s)
+{
+ struct comedi_cmd *cmd = &s->async->cmd;
+
+ return comedi_bytes_per_scan_cmd(s, cmd);
+}
EXPORT_SYMBOL_GPL(comedi_bytes_per_scan);

static unsigned int __comedi_nscans_left(struct comedi_subdevice *s,
--- a/drivers/staging/comedi/drivers/ni_mio_common.c
+++ b/drivers/staging/comedi/drivers/ni_mio_common.c
@@ -3516,6 +3516,7 @@ static int ni_cdio_check_chanlist(struct
static int ni_cdio_cmdtest(struct comedi_device *dev,
struct comedi_subdevice *s, struct comedi_cmd *cmd)
{
+ unsigned int bytes_per_scan;
int err = 0;
int tmp;

@@ -3545,9 +3546,12 @@ static int ni_cdio_cmdtest(struct comedi
err |= comedi_check_trigger_arg_is(&cmd->convert_arg, 0);
err |= comedi_check_trigger_arg_is(&cmd->scan_end_arg,
cmd->chanlist_len);
- err |= comedi_check_trigger_arg_max(&cmd->stop_arg,
- s->async->prealloc_bufsz /
- comedi_bytes_per_scan(s));
+ bytes_per_scan = comedi_bytes_per_scan_cmd(s, cmd);
+ if (bytes_per_scan) {
+ err |= comedi_check_trigger_arg_max(&cmd->stop_arg,
+ s->async->prealloc_bufsz /
+ bytes_per_scan);
+ }

if (err)
return 3;

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