Re: EXTERNAL - [PATCH v1] iio: backend: fix out-of-bound write

[Markus Burri <markus.burri@mt.com>]

On Fri, May 02, 2025 at 04:12:04PM +0100, Nuno Sá wrote:
> On Thu, 2025-05-01 at 08:32 +0200, Markus Burri wrote:
> > The buffer is set to 80 character. If a caller write more characters,
> > count is truncated to the max available space in "simple_write_to_buffer".
> > But afterwards a string terminator is written to the buffer at offset count
> > without boundary check. The zero termination is written OUT-OF-BOUND.
> > 
> > Add a check that the given buffer is smaller then the buffer to prevent.
> > 
> > Fixes: 035b4989211d ("iio: backend: make sure to NULL terminate stack buffer")
> > Signed-off-by: Markus Burri <markus.burri@mt.com>
> > ---
> >  drivers/iio/industrialio-backend.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/iio/industrialio-backend.c b/drivers/iio/industrialio-
> > backend.c
> > index a43c8d1bb3d0..3878bd698c98 100644
> > --- a/drivers/iio/industrialio-backend.c
> > +++ b/drivers/iio/industrialio-backend.c
> > @@ -155,6 +155,9 @@ static ssize_t iio_backend_debugfs_write_reg(struct file
> > *file,
> >  	ssize_t rc;
> >  	int ret;
> >  
> > +	if (count >= sizeof(buf))
> > +		return -ENOSPC;
> > +
> 
> Oh, this can indeed easily lead to an oob access. However, I would likely not
> mind in early returning an error. This is to write registers so 80 should be
> more than enough. Meaning that to trigger this, it has to be intentional. That
> said, of course we should not let that happen but I would still truncate things
> and let it fail afterwards (keep the code slightly simpler with one less check).
> 
Thanks for your response.
I would prefer the upfront error check. The code is cleaner and simpler to read.
As you wrote the buffer should only contain a register and a value and
therefore never extend the 80 character.
If there are more, it must be intentional or by mistake. In both cases I expect
to get an error back, instead of try to handle partial/wrong data.

> So I would instead do:
> 
> buf[rc] = '\0';
> 
> Thanks for catching this!
> - Nuno Sá
> 
> 
> >  	rc = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,
> > count);
> >  	if (rc < 0)
> >  		return rc;
> > 
> > base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e