linux-kernel - Re: [PATCH] kthread: potential dereference of null pointer

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Message-Id: <20211210131655.2eeb8f866382c324b605a6ce@linux-foundation.org>
Date:   Fri, 10 Dec 2021 13:16:55 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jiasheng Jiang <jiasheng@...as.ac.cn>
Cc:     pmladek@...e.com, valentin.schneider@....com, peterz@...radead.org,
        keescook@...omium.org, robdclark@...omium.org,
        samitolvanen@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kthread: potential dereference of null pointer

On Thu,  9 Dec 2021 14:43:14 +0800 Jiasheng Jiang <jiasheng@...as.ac.cn> wrote:

> The return value of kzalloc() needs to be checked.
> To avoid use of null pointer in case of the failure of alloc.
> 
> ...
>
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -101,6 +101,8 @@ void set_kthread_struct(struct task_struct *p)
>  		return;
>  
>  	kthread = kzalloc(sizeof(*kthread), GFP_KERNEL);
> +	if (!kthread)
> +		return;
>  	/*
>  	 * We abuse ->set_child_tid to avoid the new member and because it
>  	 * can't be wrongly copied by copy_process(). We also rely on fact

No, we shouldn't simply leave ->set_child_tid uninitialized if kmalloc
failed.

set_ktread_struct() appears to be designed so that callers must check
that to_kthread() returns non-zero after having called
set_kthread_struct().

Which is a quite weird interface, but I'm not seeing any bugs here.  If
kthread() sees to_kthread()==NULL then this kthread won't be created
and all the other unchecked calls to to_kthread() will never execute,
because this kthread doesn't exist.

The exception is in init_idle(), but that's __init code, executed at
boot time when we assume that allocations will always succeed.