[PATCH v2] comedi: fix divide-by-zero in comedi_buf_munge()

[Deepanshu Kartikey <kartikey406@gmail.com>]

The comedi_buf_munge() function performs a modulo operation 
`async->munge_chan %= async->cmd.chanlist_len` without first
checking if chanlist_len is zero. If a user program submits a command with
chanlist_len set to zero, this causes a divide-by-zero error when the device
processes data in the interrupt handler path.

Add a check for zero chanlist_len at the beginning of the
function, similar to the existing checks for !map and
CMDF_RAWDATA flag. When chanlist_len is zero, update
munge_count and return early, indicating the data was
handled without munging.

This prevents potential kernel panics from malformed user commands.

Reported-by: syzbot+f6c3c066162d2c43a66c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f6c3c066162d2c43a66c
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
v2: Merged the chanlist_len check with existing early return
    check as suggested by Ian Abbott

---
 drivers/comedi/comedi_buf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/comedi/comedi_buf.c b/drivers/comedi/comedi_buf.c
index 002c0e76baff..c7c262a2d8ca 100644
--- a/drivers/comedi/comedi_buf.c
+++ b/drivers/comedi/comedi_buf.c
@@ -317,7 +317,7 @@ static unsigned int comedi_buf_munge(struct comedi_subdevice *s,
 	unsigned int count = 0;
 	const unsigned int num_sample_bytes = comedi_bytes_per_sample(s);
 
-	if (!s->munge || (async->cmd.flags & CMDF_RAWDATA)) {
+	if (!s->munge || (async->cmd.flags & CMDF_RAWDATA) || async->cmd.chanlist_len == 0) {
 		async->munge_count += num_bytes;
 		return num_bytes;
 	}
-- 
2.43.0