linux-kernel - Re: [PATCH v2] tee: optee: Fix supplicant wait loop

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Message-Id: <15a801d6-2274-4772-bbe3-5d2083b91b64@app.fastmail.com>
Date: Mon, 03 Feb 2025 09:16:05 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Sumit Garg" <sumit.garg@...aro.org>,
 "Jens Wiklander" <jens.wiklander@...aro.org>
Cc: op-tee@...ts.trustedfirmware.org,
 "Jerome Forissier" <jerome.forissier@...aro.org>, dannenberg@...com,
 javier@...igon.com, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] tee: optee: Fix supplicant wait loop

On Mon, Feb 3, 2025, at 09:00, Sumit Garg wrote:
> OP-TEE supplicant is a user-space daemon and it's possible for it
> being hung or crashed or killed in the middle of processing an OP-TEE
> RPC call. It becomes more complicated when there is incorrect shutdown
> ordering of the supplicant process vs the OP-TEE client application which
> can eventually lead to system hang-up waiting for the closure of the
> client application.
>
> Allow the client process waiting in kernel for supplicant response to
> be killed rather than indefinitetly waiting in an unkillable state.

It would be good to mention that the existing version ends up in
a busy-loop here because of the broken wait_for_completion_interruptible()
loop.

A normal uninterruptible wait should not have resulted in the hung-task
watchdog getting triggered, but the endless loop would.

> +	if (wait_for_completion_killable(&req->c)) {
> +		if (!mutex_lock_killable(&supp->mutex))	{
>  			if (req->in_queue) {

Using mutex_trylock() here is probably clearer than
mutex_lock_killable(), since a task that is already in the
process of getting killed won't ever wait for the mutex.
mutex_lock_killable() does try to get the lock first though
if nobody else holds it already, which is the same as trylock.

     Arnd