linux-kernel - Re: [PATCH] um: protect find_task_by_pid

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Date:   Sun, 16 Oct 2022 19:52:48 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Dawei Li <set_pte_at@...look.com>, richard@....at,
        anton.ivanov@...bridgegreys.com
Cc:     linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] um: protect find_task_by_pid_ns() with rcu lock

On Sun, 2022-10-16 at 23:10 +0800, Dawei Li wrote:
> find_task_by_pid_ns() is safe if and only if it's under protection
> from rcu read lock, implements the protection.
> 
> base-commit: 193cb8372424184dde28088a4230a5fed0afb0ad
> 
> Signed-off-by: Dawei Li <set_pte_at@...look.com>
> ---
>  arch/um/drivers/mconsole_kern.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
> index 5026e7b9adfe..6b80e766ae80 100644
> --- a/arch/um/drivers/mconsole_kern.c
> +++ b/arch/um/drivers/mconsole_kern.c
> @@ -683,7 +683,10 @@ void mconsole_stack(struct mc_request *req)
>  		return;
>  	}
>  
> +	rcu_read_lock();
>  	to = find_task_by_pid_ns(pid_requested, &init_pid_ns);
> +	rcu_read_unlock();
> +
>  	if ((to == NULL) || (pid_requested == 0)) {
>  		mconsole_reply(req, "Couldn't find that pid", 1, 0);
>  		return;

This can't be right - the 'to' pointer is used later in the code (not in
the existing context), so the protection, if needed, would have to
include the later "with_console()" call?

johannes