linux-kernel - Re: [PATCH] livepatch: add locking to force and signal functions

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Date:   Wed, 20 Dec 2017 14:08:16 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jpoimboe@...hat.com, jeyu@...nel.org, jikos@...nel.org,
        jbaron@...mai.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: add locking to force and signal functions

On Wed 2017-12-20 10:28:07, Miroslav Benes wrote:
> klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
> because it seemed to be superfluous. A potential race in
> klp_send_signals() was harmless and there was nothing in
> klp_force_transition() which needed to be synchronized. That changed
> with the addition of klp_forced variable during the review process.
> 
> There is a small window now, when klp_complete_transition() does not see
> klp_forced set to true while all tasks have been already transitioned to
> the target state. module_put() is called and the module can be removed.
> 
> Acquire klp_mutex to prevent it. Do the same in klp_send_signals() just
> to be sure. There is no real downside to that.
> 
> Reported-by: Jason Baron <jbaron@...mai.com>
> Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> ---
>  kernel/livepatch/transition.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index be5bfa533ee8..3f932ff607cd 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -625,6 +625,8 @@ void klp_send_signals(void)
>  
>  	pr_notice("signaling remaining tasks\n");
>  
> +	mutex_lock(&klp_mutex);
> +
>  	read_lock(&tasklist_lock);
>  	for_each_process_thread(g, task) {
>  		if (!klp_patch_pending(task))
> @@ -653,6 +655,8 @@ void klp_send_signals(void)
>  		}
>  	}
>  	read_unlock(&tasklist_lock);
> +
> +	mutex_unlock(&klp_mutex);

It would be cleaner if the lock guarded also the check:

	if (patch != klp_transition_patch)
		return -EINVAL;

in signal_store(). Then we could remove also the comment
above this check.

Same is true also for the force part stuff.

Best Regards,
Petr

PS: I am sorry that I hand waved the proposed solution when
we spoke about it yeasterday. I should have looked into
the code.