comedi: fix divide-by-zero in comedi_buf_munge()

commit 87b318ba81dda2ee7b603f4f6c55e78ec3e95974 upstream.

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>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://patch.msgid.link/20250924102639.1256191-1-kartikey406@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/comedi/comedi_buf.c b/drivers/comedi/comedi_buf.c
index 393966c097405d16717d6712bd5673699f08407e..ef38e9ad98f62b0d90670e490047933706f71628 100644 (file)
--- a/drivers/comedi/comedi_buf.c
+++ b/drivers/comedi/comedi_buf.c
@@ -368,7 +368,7 @@ 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;
        }