linux-kernel - Re: [PATCH] fs/proc: Fix NULL pointer dereference in pid_delete

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Date:   Wed, 9 Dec 2020 13:16:52 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Yahu Gao <yahu.gao@...driver.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs/proc: Fix NULL pointer dereference in
 pid_delete_dentry

On Wed, Dec 09, 2020 at 07:21:00PM +0800, Yahu Gao wrote:
> Get the staus of task from the pointer of proc inode directly is not
> safe. The function get_proc_task make it happen in RCU protection.

This is completely broken,  get_proc_task() acquires a reference to
task_struct; your patch is an instant leak.

> Signed-off-by: Yahu Gao <yahu.gao@...driver.com>
> ---
>  fs/proc/base.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 1bc9bcdef09f..05f33bb35067 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1994,7 +1994,7 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
>  
>  static inline bool proc_inode_is_dead(struct inode *inode)
>  {
> -	return !proc_pid(inode)->tasks[PIDTYPE_PID].first;
> +	return !get_proc_task(inode);
>  }
>  
>  int pid_delete_dentry(const struct dentry *dentry)
> -- 
> 2.25.1
>