Re: [PATCH] [RFC] iio: pressure: dlhl60d: Check mask_width for IRQs

[Jonathan Cameron]

On Fri, 23 Feb 2024 09:14:53 -0800
Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> On Fri, Feb 23, 2024 at 05:09:18PM +0000, Jonathan Cameron wrote:
> > On Thu, 22 Feb 2024 14:23:39 -0800
> > Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >   
> > > Clang tripped over a FORTIFY warning in this code, and while it seems it
> > > may be a false positive in Clang due to loop unwinding, the code in
> > > question seems to make a lot of assumptions.   
> > 
> > Hi Kees,
> > 
> > The assumptions are mostly characteristics of how the IIO buffers work
> > with the scan masks defined based on indexes in the driver provided
> > struct iio_chan_spec arrays.
> > 
> > This driver is doing more work than it should need to as we long ago
> > moved some of the more fiddly handling into the IIO core.
> >   
> > > Comments added, and the
> > > Clang warning[1] has been worked around by growing the array size.
> > > Also there was an uninitialized 4th byte in the __be32 array that was
> > > being sent through to iio_push_to_buffers().  
> > 
> > That is indeed not good - the buffer should have been zero initialized.  
> 
> Okay, I'll get this respun and include the fix.
> 
> >   
> > > 
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/2000 [1]
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > > Cc: Jonathan Cameron <jic23@xxxxxxxxxx>
> > > Cc: Lars-Peter Clausen <lars@xxxxxxxxxx>
> > > Cc: "Uwe Kleine-König" <u.kleine-koenig@xxxxxxxxxxxxxx>
> > > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > > Cc: "Nuno Sá" <nuno.sa@xxxxxxxxxx>
> > > Cc: linux-iio@xxxxxxxxxxxxxxx
> > > ---
> > >  drivers/iio/pressure/dlhl60d.c | 11 +++++++++--
> > >  1 file changed, 9 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/iio/pressure/dlhl60d.c b/drivers/iio/pressure/dlhl60d.c
> > > index 28c8269ba65d..9bbecd0bfe88 100644
> > > --- a/drivers/iio/pressure/dlhl60d.c
> > > +++ b/drivers/iio/pressure/dlhl60d.c
> > > @@ -250,20 +250,27 @@ static irqreturn_t dlh_trigger_handler(int irq, void *private)
> > >  	struct dlh_state *st = iio_priv(indio_dev);
> > >  	int ret;
> > >  	unsigned int chn, i = 0;
> > > -	__be32 tmp_buf[2];
> > > +	/* This was only an array pair of 4 bytes. */  
> > 
> > True, which is the right size as far as I can tell.
> > If we need this to suppress a warning then comment should say that.  
> 
> Okay. I think I'll leave it as 2 and manually "unroll" the loop.

Without the available mask that is a little fiddly you'll have
deal with  channel 0 only enabled, channel 1 only enabled and
both channels 0 and channel 1 enabled.
Not too bad though as only 2 channels.
 
> 
> >   
> > > +	__be32 tmp_buf[4] = { };
> > >  
> > >  	ret = dlh_start_capture_and_read(st);
> > >  	if (ret)
> > >  		goto out;
> > >  
> > > +	/* Nothing was checking masklength vs ARRAY_SIZE(tmp_buf)? */  
> > 
> > Not needed but no way a compiler could know that.
> >   
> > > +	if (WARN_ON_ONCE(indio_dev->masklength > ARRAY_SIZE(tmp_buf)))
> > > +		goto out;
> > > +
> > >  	for_each_set_bit(chn, indio_dev->active_scan_mask,  
> > 
> > This is all a bit pointless if not 'wrong' other than the
> > 4th byte uninitialized part.  The limit can be hard coded as 2 as
> > that's a characteristic of this driver.
> > 
> > For device that always read a particular set of channels they
> > should provide indio_dev->available_scan_masks = { BIT(1) | BIT(0), 0 };
> > and then always push all the data making this always
> > 
> > 	memcpy(&tmp_buf[0], &st->rx_buf[1], 3);
> > 	mempcy(&tmp_buf[1], &st->rx_buf[1] + 3, 3);  
> 
> Okay, so this could be unrolled manually to check just for bits 0 and 1?

Ideally it wouldn't check them - the hardwork has been done to read both
channels anyway and the IIO core handles userspace or in kernel consumers
that want a subset of what is enabled, but that needs the available_scan_masks
to be set so that the IIO core knows all channels always enabled.

> 
> > 
> > The buffer demux code in the IIO core will deal with repacking the data
> > if only one channel is enabled.
> >   
> > >  		indio_dev->masklength) {
> > > -		memcpy(tmp_buf + i,
> > > +		/* This is copying 3 bytes. What about the 4th? */
> > > +		memcpy(&tmp_buf[i],
> > >  			&st->rx_buf[1] + chn * DLH_NUM_DATA_BYTES,
> > >  			DLH_NUM_DATA_BYTES);
> > >  		i++;
> > >  	}
> > >  
> > > +	/* How do we know the iio buffer_list has only 2 items? */  
> > 
> > Can only include items from the channels array at indexes up to the max
> > scan_index in there, so 0 and 1 in this case (1 might not be present if only
> > one channel is enabled). Sizes (and alignment) are given by storagebits so
> > 4 bytes for each.  
> 
> This code pattern seems repeated through all of iio, so I guess we'll
> leave it as-is. It seems like it'd be nice to have a "length" argument
> to iio_push_to_buffers(), just to sanity check, but that would need to
> be a pretty large patch. :P

yeah. Hindsight!

We could add it in an incremental fashion though
iio_push_to_bufs(struct iio_dev *indio_dev, void *buf, size_t buf_len)
with a length parameter.  The oddity that is
iio_push_to_buffers_with_timestamp() would benefit here as that needs
a bigger buffer than immediately apparent in the driver and we've
had a few bugs around that over the years.

It would probably be a one way check.
I might have a play and see how useful this would be.

> 
> >   
> > >  	iio_push_to_buffers(indio_dev, tmp_buf);
> > >  
> > >  out:  
> 
> Thanks for looking at this!
> 
> -Kees
>