[PATCH] iio: dac: ad3552r-hs: fix out-of-bound write in ad3552r_hs_write_data_source

[Miaoqian Lin <linmq006@gmail.com>]

When simple_write_to_buffer() succeeds, it returns the number of bytes
actually copied to the buffer, which may be less than the requested
'count' if the buffer size is insufficient. However, the current code
incorrectly uses 'count' as the index for null termination instead of
the actual bytes copied, leading to out-of-bound write.

Add a check for the count and use the return value as the index.

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>
---
 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)