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 | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/industrialio-backend.c b/drivers/iio/industrialio-backend.c
index a43c8d1bb3d0..31fe793e345e 100644
--- a/drivers/iio/industrialio-backend.c
+++ b/drivers/iio/industrialio-backend.c
@@ -155,11 +155,14 @@ static ssize_t iio_backend_debugfs_write_reg(struct file *file,
 	ssize_t rc;
 	int ret;
 
+	if (count >= sizeof(buf))
+		return -ENOSPC;
+
 	rc = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf, count);
 	if (rc < 0)
 		return rc;
 
-	buf[count] = '\0';
+	buf[rc] = '\0';
 
 	ret = sscanf(buf, "%i %i", &back->cached_reg_addr, &val);
 

base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e
-- 
2.39.5

On Mon,  5 May 2025 06:53:46 +0200
Markus Burri <markus.burri@mt.com> 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 | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/industrialio-backend.c b/drivers/iio/industrialio-backend.c
> index a43c8d1bb3d0..31fe793e345e 100644
> --- a/drivers/iio/industrialio-backend.c
> +++ b/drivers/iio/industrialio-backend.c
> @@ -155,11 +155,14 @@ static ssize_t iio_backend_debugfs_write_reg(struct file *file,
>  	ssize_t rc;
>  	int ret;
>  
> +	if (count >= sizeof(buf))
> +		return -ENOSPC;

if count is buf - 1 this passes but...

> +
>  	rc = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf, count);

This effectively copied in one less than we expect so we drop a character.
So I think we want to add the -1 to the check as Nuno suggested.

>  	if (rc < 0)
>  		return rc;
>  
> -	buf[count] = '\0';
> +	buf[rc] = '\0';
>  
>  	ret = sscanf(buf, "%i %i", &back->cached_reg_addr, &val);
>  
> 
> base-commit: b4432656b36e5cc1d50a1f2dc15357543add530e