[PATCH net-next 0/2] can: publish tty->disc_data only after register_candev()

[PATCH net-next 0/2] can: publish tty->disc_data only after register_candev()

[Chaosheng Qu]

slcan_open() and can327_ldisc_open() publish the private structure to the
tty layer through tty->disc_data and register the net_device afterwards.
If register_candev() fails, free_candev() releases the net_device and the
private area with it, while tty->disc_data still points at the freed
memory. register_candev() fails with -ENOMEM from register_netdevice()
when the device name node or the kobject cannot be allocated.

Both patches publish the pointer only after the device is registered, so
the error path has nothing to unwind.

The slcan side is a regression. From the driver's addition in 2010 until
the 2022 rewrite the error path cleared the pointer in err_free_chan;
commit cfcb4465e992 ("can: slcan: remove legacy infrastructure") folded
that label away and dropped the tty->disc_data = NULL with it. can327 was
written later against this tree and never had the cleanup.

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 series
removes a trap for the next modification of these functions rather than
fixing a bug that can be hit now.

Both patches apply with git am to mkl/linux-can-next testing and build
without new warnings on x86_64 with CAN_SLCAN and CAN_CAN327 enabled;
checkpatch.pl --strict is clean. The success path was tested by attaching
N_SLCAN and N_CAN327 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

Chaosheng Qu (2):
  can: slcan: publish tty->disc_data only after register_candev()
  can: can327: publish tty->disc_data only after register_candev()

 drivers/net/can/can327.c           | 3 ++-
 drivers/net/can/slcan/slcan-core.c | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

-- 
2.43.0

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

[Chaosheng Qu]

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

[PATCH net-next 2/2] can: can327: publish tty->disc_data only after register_candev()

[Chaosheng Qu]

can327_ldisc_open() publishes the private structure to the tty layer
before registering the net_device:

	tty->disc_data = elm;

	err = register_candev(elm->dev);
	if (err) {
		free_candev(elm->dev);
		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 can327 structure in its private area, while
tty->disc_data still points at the freed memory.

Publish the pointer only after the device is registered, so the error
path has nothing to unwind. slcan does the same after its own patch, as
do ppp_async and nci_uart.

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_CAN327 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/can327.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/can327.c b/drivers/net/can/can327.c
index 90f5e35f3c8f..d40dadbb5733 100644
--- a/drivers/net/can/can327.c
+++ b/drivers/net/can/can327.c
@@ -1037,7 +1037,6 @@ static int can327_ldisc_open(struct tty_struct *tty)
 
 	/* Mark ldisc channel as alive */
 	elm->tty = tty;
-	tty->disc_data = elm;
 
 	/* Let 'er rip */
 	err = register_candev(elm->dev);
@@ -1046,6 +1045,8 @@ static int can327_ldisc_open(struct tty_struct *tty)
 		return err;
 	}
 
+	tty->disc_data = elm;
+
 	netdev_info(elm->dev, "can327 on %s.\n", tty->name);
 
 	return 0;
-- 
2.43.0