Re: [PATCH] comedi: adl_pci9111: Fix possible division by zero in pci9111_ai_do_cmd_test()

[Ian Abbott]

On 18/09/2024 12:41, Ian Abbott wrote:
> On 18/09/2024 11:43, Aleksandr Mishin wrote:
> > Division by zero is possible in pci9111_ai_do_cmd_test() in case of scan
> > begin trigger source is TRIG_TIMER and either 'chanlist_len' or
> > 'convert_arg' is zero.
> >
> > Add zero value check to prevent division by zero.
> >
> > Found by Linux Verification Center (linuxtesting.org) with SVACE.
> >
> > Fixes: f1c51faabc4d ("staging: comedi: adl_pci9111: tidy up 
> > (*do_cmdtest) Step 4")
> > Signed-off-by: Aleksandr Mishin <amishin@xxxxxxxxxx>
> > ---
> >   drivers/comedi/drivers/adl_pci9111.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/comedi/drivers/adl_pci9111.c 
> > b/drivers/comedi/drivers/adl_pci9111.c
> > index 086d93f40cb9..ec1fb570b98c 100644
> > --- a/drivers/comedi/drivers/adl_pci9111.c
> > +++ b/drivers/comedi/drivers/adl_pci9111.c
> > @@ -312,6 +312,8 @@ static int pci9111_ai_do_cmd_test(struct 
> > comedi_device *dev,
> >        */
> >       if (cmd->scan_begin_src == TRIG_TIMER) {
> >           arg = cmd->chanlist_len * cmd->convert_arg;
> > +        if (!arg)
> > +            return 4;
> >           if (arg < cmd->scan_begin_arg)
> >               arg *= (cmd->scan_begin_arg / arg);
>
> Nice catch!  `cmd->convert_arg` will be non-zero due to earlier checks, 
> but `cmd->chanlist_len` could be zero for the `COMEDI_CMDTEST` ioctl. 
> (The function is called for the `COMEDI_CMDTEST` and `COMEDI_CMD` 
> ioctls, but only `COMEDI_CMD` checks that `chanlist_len` is non-zero 
> before calling the function.)
>
> Reviewed-by: Ian Abbott <abbotti@xxxxxxxxx>

On second thoughts, the code should not return 4 (indicating a problem 
at checking step 4) here if `chanlist_len` is 0, because it is OK to use 
a zero `chanlist_len` value when using the `COMEDI_CMDTEST` ioctl.

Suggested fix is to keep your change, but to combine it with this change:

-	if (cmd->scan_begin_src == TRIG_TIMER) {
+	if (cmd->scan_begin_src == TRIG_TIMER && cmd->chanlist_len) {

I think `cmd->convert_arg` will be non-zero unless the call to 
`comedi_8254_cascade_ns_to_timer()` somehow manages to set it to zero.

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