[PATCH v2] pps: reject N_PPS on pseudo-terminals

[Hungyu Lin <dennylin0707@gmail.com>]

Attaching N_PPS to a PTY registers a PPS source even though PTYs do not
report DCD changes. An unprivileged user can exhaust the PPS source IDs
this way, causing registration of a real PPS source to fail with
-EBUSY.

Reject N_PPS on PTYs before allocating a source.

Fixes: a0880df0ccde ("pps: serial clients support")
Suggested-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
---
Changes in v2:
- Reject PTYs before registering a PPS source, as suggested by Rodolfo.
- Drop the stable Cc.

v1: https://lore.kernel.org/r/20261107211753.75932-1-dennylin0707@gmail.com

 drivers/pps/clients/pps-ldisc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/pps/clients/pps-ldisc.c b/drivers/pps/clients/pps-ldisc.c
index fa5660f3c4b7..83f4caced0c9 100644
--- a/drivers/pps/clients/pps-ldisc.c
+++ b/drivers/pps/clients/pps-ldisc.c
@@ -46,6 +46,9 @@ static int pps_tty_open(struct tty_struct *tty)
 	struct pps_device *pps;
 	int ret;
 
+	if (drv->type == TTY_DRIVER_TYPE_PTY)
+		return -EINVAL;
+
 	info.owner = THIS_MODULE;
 	info.dev = NULL;
 	snprintf(info.name, PPS_MAX_NAME_LEN, "%s%d", drv->driver_name, index);
-- 
2.50.1 (Apple Git-155)