Re: [PATCH 2/2] serdev: use tty_port_tty_get() in ttyport_write_buf() to prevent UAF

[Markus Probst <markus.probst@posteo.de>]

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On Fri, 2026-07-31 at 10:06 +0200, Greg Kroah-Hartman wrote:
> From: Joshua Rogers <linux@joshua.hu>
> 
> ttyport_write_buf() snapshots serport->tty as a raw pointer without
> taking a reference, while ttyport_close() can concurrently release the
> tty via tty_release_struct(), leading to a use-after-free. Use
> tty_port_tty_get() to obtain a reference-counted tty pointer, matching
> the pattern already used by ttyport_write_wakeup().
> 
> Assisted-by: AISLE:Snapshot
> Cc: stable <stable@kernel.org>
> Signed-off-by: Joshua Rogers <linux@joshua.hu>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  drivers/tty/serdev/serdev-ttyport.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
> index 48ce5b3f8308..4d37c8130dd7 100644
> --- a/drivers/tty/serdev/serdev-ttyport.c
> +++ b/drivers/tty/serdev/serdev-ttyport.c
> @@ -85,13 +85,21 @@ static const struct tty_port_client_operations client_ops = {
>  static ssize_t ttyport_write_buf(struct serdev_controller *ctrl, const u8 *data, size_t len)
>  {
>  	struct serport *serport = serdev_controller_get_drvdata(ctrl);
> -	struct tty_struct *tty = serport->tty;
> +	struct tty_struct *tty;
> +	ssize_t ret;
>  
>  	if (!test_bit(SERPORT_ACTIVE, &serport->flags))
>  		return 0;
>  
> +	tty = tty_port_tty_get(serport->port);
> +	if (!tty)
> +		return 0;
> +
>  	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
> -	return tty->ops->write(serport->tty, data, len);
> +	ret = tty->ops->write(tty, data, len);
> +	tty_kref_put(tty);
> +
> +	return ret;
>  }
>  
>  static void ttyport_write_flush(struct serdev_controller *ctrl)

It should be the responsibility of the driver to ensure
`serdev_device_close` and `serdev_device_write_buf` are not called
concurrently.

This also applies to
- serdev_device_set_baudrate
- serdev_device_set_flow_control
- serdev_device_wait_until_sent
and more, which currently iirc even introduce a null pointer
dereference if called with the serdev device closed.

I don't see why `serdev_device_write_buf` needs special protection
here, which the other functions don't need for some reason.

As with the previous patch, this still makes sense for hardening.
With scope_guard being used,

Reviewed-by: Markus Probst <markus.probst@posteo.de>


Thanks
- Markus Probst


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