[PATCH net-next 1/2] can: slcan: publish tty->disc_data only after register_candev()

[Chaosheng Qu <chaosheng.qu.linux@gmail.com>]

slcan_open() publishes the private structure to the tty layer before
registering the net_device:

	tty->disc_data = sl;

	err = register_candev(dev);
	if (err) {
		free_candev(dev);
		pr_err("can't register candev\n");
		return err;
	}

register_candev() fails with -ENOMEM from register_netdevice() when the
device name node or the kobject cannot be allocated. The error path then
frees the net_device and the slcan structure in its private area, while
tty->disc_data still points at the freed memory.

This is a regression from the 2022 rewrite. From the driver's addition in
2010 until then the error path cleared the pointer:

	err_free_chan:
		sl->tty = NULL;
		tty->disc_data = NULL;
		clear_bit(SLF_INUSE, &sl->flags);

Commit cfcb4465e992 ("can: slcan: remove legacy infrastructure") folded
that label away and dropped the tty->disc_data = NULL with it.

Publish the pointer only after the device is registered, so the error
path has nothing to unwind. ppp_async and nci_uart assign it the same
way; nci_uart was changed to that in commit fc27ab48904c ("NFC: nci:
uart: Set tty->disc_data only in success path").

The stale pointer is not observable today: tty_set_termios_ldisc() clears
tty->disc_data before .open() runs and again when a failed ldisc change
falls back, and the ldisc entry points are serialised against .open() by
tty_ldisc_ref() (async) and tty_ldisc_ref_wait() (syscall). The change
removes a trap for the next modification of these functions.

The success path was tested by attaching N_SLCAN to a pty under QEMU: the
ldisc opens, creates the net_device and SIOCGIFNAME returns its name. The
register_candev() failure path cannot be forced from userspace and is
only compile tested.

Assisted-by: LLM
Signed-off-by: Chaosheng Qu <chaosheng.qu.linux@gmail.com>
---
 drivers/net/can/slcan/slcan-core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/slcan/slcan-core.c b/drivers/net/can/slcan/slcan-core.c
index c9414faafe66..fc6ecf110aab 100644
--- a/drivers/net/can/slcan/slcan-core.c
+++ b/drivers/net/can/slcan/slcan-core.c
@@ -849,7 +849,6 @@ static int slcan_open(struct tty_struct *tty)
 
 	/* Mark ldisc channel as alive */
 	sl->tty = tty;
-	tty->disc_data = sl;
 
 	err = register_candev(dev);
 	if (err) {
@@ -858,6 +857,8 @@ static int slcan_open(struct tty_struct *tty)
 		return err;
 	}
 
+	tty->disc_data = sl;
+
 	netdev_info(dev, "slcan on %s.\n", tty->name);
 	/* TTY layer expects 0 on success */
 	return 0;
-- 
2.43.0