When simple_write_to_buffer() succeeds, it returns the number of bytes
actually copied to the buffer. The code incorrectly uses 'count'
as the index for null termination instead of the actual bytes copied.
If count exceeds the buffer size, this leads to out-of-bounds write.
Add a check for the count and use the return value as the index.

The bug was validated using a demo module that mirrors the original
code and was tested under QEMU.

Pattern of the bug:
- A fixed 64-byte stack buffer is filled using count.
- If count > 64, the code still does buf[count] = '\0', causing an
- out-of-bounds write on the stack.

Steps for reproduce:
- Opens the device node.
- Writes 128 bytes of A to it.
- This overflows the 64-byte stack buffer and KASAN reports the OOB.

Found via static analysis. This is similar to the
commit da9374819eb3 ("iio: backend: fix out-of-bound write")

Fixes: b1c5d68ea66e ("iio: dac: ad3552r-hs: add support for internal ramp")
Cc: stable@vger.kernel.org
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
---
changes in v2:
- update commit message
- v1 link: https://lore.kernel.org/all/20251027150713.59067-1-linmq006@gmail.com/
---
 drivers/iio/dac/ad3552r-hs.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/dac/ad3552r-hs.c b/drivers/iio/dac/ad3552r-hs.c
index 41b96b48ba98..a9578afa7015 100644
--- a/drivers/iio/dac/ad3552r-hs.c
+++ b/drivers/iio/dac/ad3552r-hs.c
@@ -549,12 +549,15 @@ static ssize_t ad3552r_hs_write_data_source(struct file *f,
 
 	guard(mutex)(&st->lock);
 
+	if (count >= sizeof(buf))
+		return -ENOSPC;
+
 	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,
 				     count);
 	if (ret < 0)
 		return ret;
 
-	buf[count] = '\0';
+	buf[ret] = '\0';
 
 	ret = match_string(dbgfs_attr_source, ARRAY_SIZE(dbgfs_attr_source),
 			   buf);
-- 
2.39.5 (Apple Git-154)

On Wed,  7 Jan 2026 22:35:50 +0800
Miaoqian Lin <linmq006@gmail.com> wrote:

> When simple_write_to_buffer() succeeds, it returns the number of bytes
> actually copied to the buffer. The code incorrectly uses 'count'
> as the index for null termination instead of the actual bytes copied.
> If count exceeds the buffer size, this leads to out-of-bounds write.
> Add a check for the count and use the return value as the index.
> 
> The bug was validated using a demo module that mirrors the original
> code and was tested under QEMU.
> 
> Pattern of the bug:
> - A fixed 64-byte stack buffer is filled using count.
> - If count > 64, the code still does buf[count] = '\0', causing an
> - out-of-bounds write on the stack.
> 
> Steps for reproduce:
> - Opens the device node.
> - Writes 128 bytes of A to it.
> - This overflows the 64-byte stack buffer and KASAN reports the OOB.
> 
> Found via static analysis. This is similar to the
> commit da9374819eb3 ("iio: backend: fix out-of-bound write")
> 
> Fixes: b1c5d68ea66e ("iio: dac: ad3552r-hs: add support for internal ramp")
> Cc: stable@vger.kernel.org
> Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Hi Miaoquian,

You haven't listed why you didn't add David's Reviewed-by tag from v1.
b4 found it though so all is fine, but none the less please make sure you either
pick up tags, or in the change log say that you didn't deliberately.

Applied to the fixes-togreg branch of iio.git

Thanks,

Jonathan

> ---
> changes in v2:
> - update commit message
> - v1 link: https://lore.kernel.org/all/20251027150713.59067-1-linmq006@gmail.com/
> ---
>  drivers/iio/dac/ad3552r-hs.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/dac/ad3552r-hs.c b/drivers/iio/dac/ad3552r-hs.c
> index 41b96b48ba98..a9578afa7015 100644
> --- a/drivers/iio/dac/ad3552r-hs.c
> +++ b/drivers/iio/dac/ad3552r-hs.c
> @@ -549,12 +549,15 @@ static ssize_t ad3552r_hs_write_data_source(struct file *f,
>  
>  	guard(mutex)(&st->lock);
>  
> +	if (count >= sizeof(buf))
> +		return -ENOSPC;
> +
>  	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,
>  				     count);
>  	if (ret < 0)
>  		return ret;
>  
> -	buf[count] = '\0';
> +	buf[ret] = '\0';
>  
>  	ret = match_string(dbgfs_attr_source, ARRAY_SIZE(dbgfs_attr_source),
>  			   buf);

On Wed, 2026-01-07 at 22:35 +0800, Miaoqian Lin wrote:
> When simple_write_to_buffer() succeeds, it returns the number of bytes
> actually copied to the buffer. The code incorrectly uses 'count'
> as the index for null termination instead of the actual bytes copied.
> If count exceeds the buffer size, this leads to out-of-bounds write.
> Add a check for the count and use the return value as the index.
> 
> The bug was validated using a demo module that mirrors the original
> code and was tested under QEMU.
> 
> Pattern of the bug:
> - A fixed 64-byte stack buffer is filled using count.
> - If count > 64, the code still does buf[count] = '\0', causing an
> - out-of-bounds write on the stack.
> 
> Steps for reproduce:
> - Opens the device node.
> - Writes 128 bytes of A to it.
> - This overflows the 64-byte stack buffer and KASAN reports the OOB.
> 
> Found via static analysis. This is similar to the
> commit da9374819eb3 ("iio: backend: fix out-of-bound write")
> 
> Fixes: b1c5d68ea66e ("iio: dac: ad3552r-hs: add support for internal ramp")
> Cc: stable@vger.kernel.org
> Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
> ---

Reviewed-by: Nuno Sá <nuno.sa@analog.com>

> changes in v2:
> - update commit message
> - v1 link: https://lore.kernel.org/all/20251027150713.59067-1-linmq006@gmail.com/
> ---
>  drivers/iio/dac/ad3552r-hs.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/dac/ad3552r-hs.c b/drivers/iio/dac/ad3552r-hs.c
> index 41b96b48ba98..a9578afa7015 100644
> --- a/drivers/iio/dac/ad3552r-hs.c
> +++ b/drivers/iio/dac/ad3552r-hs.c
> @@ -549,12 +549,15 @@ static ssize_t ad3552r_hs_write_data_source(struct file *f,
>  
>  	guard(mutex)(&st->lock);
>  
> +	if (count >= sizeof(buf))
> +		return -ENOSPC;
> +
>  	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,
>  				     count);
>  	if (ret < 0)
>  		return ret;
>  
> -	buf[count] = '\0';
> +	buf[ret] = '\0';
>  
>  	ret = match_string(dbgfs_attr_source, ARRAY_SIZE(dbgfs_attr_source),
>  			   buf);

On Wed, Jan 07, 2026 at 10:35:50PM +0800, Miaoqian Lin wrote:
> When simple_write_to_buffer() succeeds, it returns the number of bytes
> actually copied to the buffer. The code incorrectly uses 'count'
> as the index for null termination instead of the actual bytes copied.
> If count exceeds the buffer size, this leads to out-of-bounds write.
> Add a check for the count and use the return value as the index.
> 
> The bug was validated using a demo module that mirrors the original
> code and was tested under QEMU.
> 
> Pattern of the bug:
> - A fixed 64-byte stack buffer is filled using count.
> - If count > 64, the code still does buf[count] = '\0', causing an
> - out-of-bounds write on the stack.
> 
> Steps for reproduce:
> - Opens the device node.
> - Writes 128 bytes of A to it.
> - This overflows the 64-byte stack buffer and KASAN reports the OOB.
> 
> Found via static analysis. This is similar to the
> commit da9374819eb3 ("iio: backend: fix out-of-bound write")

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>

-- 
With Best Regards,
Andy Shevchenko