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

["Nuno Sá" <noname.nuno@gmail.com>]

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).

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