linux-kernel - Re: [PATCH v1] usb: typec: tcpm: enforce ready state when queueing alt mode vdm

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Date: Thu, 25 Apr 2024 12:49:17 +0300
From: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To: RD Babiera <rdbabiera@...gle.com>
Cc: linux@...ck-us.net, gregkh@...uxfoundation.org, hdegoede@...hat.com,
	badhri@...gle.com, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1] usb: typec: tcpm: enforce ready state when queueing
 alt mode vdm

On Tue, Apr 23, 2024 at 08:23:57PM +0000, RD Babiera wrote:
> Before sending Enter Mode for an Alt Mode, there is a gap between Discover
> Modes and the Alt Mode driver queueing the Enter Mode VDM for the port
> partner to send a message to the port.
> 
> If this message results in unregistering Alt Modes such as in a DR_SWAP,
> then the following deadlock can occur with respect to the DisplayPort Alt
> Mode driver:
> 1. The DR_SWAP state holds port->lock. Unregistering the Alt Mode driver
> results in a cancel_work_sync() that waits for the current dp_altmode_work
> to finish.
> 2. dp_altmode_work makes a call to tcpm_altmode_enter. The deadlock occurs
> because tcpm_queue_vdm_unlock attempts to hold port->lock.
> 
> Before attempting to grab the lock, ensure that the port is in a state
> vdm_run_state_machine can run in. Alt Mode unregistration will not occur
> in these states.

I'm probable missing something, but wouldn't it be safer to check
port->state after grabbing the lock?

> Fixes: 03eafcfb60c0 ("usb: typec: tcpm: Add tcpm_queue_vdm_unlocked() helper")
> Cc: stable@...r.kernel.org
> Signed-off-by: RD Babiera <rdbabiera@...gle.com>
> ---
>  drivers/usb/typec/tcpm/tcpm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
> index c26fb70c3ec6..6fa1601ac259 100644
> --- a/drivers/usb/typec/tcpm/tcpm.c
> +++ b/drivers/usb/typec/tcpm/tcpm.c
> @@ -1564,6 +1564,10 @@ static void tcpm_queue_vdm(struct tcpm_port *port, const u32 header,
>  static void tcpm_queue_vdm_unlocked(struct tcpm_port *port, const u32 header,
>  				    const u32 *data, int cnt, enum tcpm_transmit_type tx_sop_type)
>  {
> +	if (port->state != SRC_READY && port->state != SNK_READY &&
> +	    port->state != SRC_VDM_IDENTITY_REQUEST)
> +		return;
> +
>  	mutex_lock(&port->lock);
>  	tcpm_queue_vdm(port, header, data, cnt, TCPC_TX_SOP);
>  	mutex_unlock(&port->lock);
> 
> base-commit: 684e9f5f97eb4b7831298ffad140d5c1d426ff27
> -- 
> 2.44.0.769.g3c40516874-goog

-- 
heikki