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Date:   Mon, 20 Jul 2020 17:00:59 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Hillf Danton <hdanton@sina.com>
Cc:     syzbot <syzbot+75867c44841cb6373570@syzkaller.appspotmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com,
        Markus Elfring <Markus.Elfring@web.de>
Subject: Re: KASAN: use-after-free Read in userfaultfd_release (2)
Message-ID: <20200720160059.GO2786714@ZenIV.linux.org.uk>
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 <20200713084512.10416-1-hdanton@sina.com>
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On Mon, Jul 13, 2020 at 04:45:12PM +0800, Hillf Danton wrote:

> Bridge the gap between slab free and the fput in task work wrt
> file's private data.

No.  This

> @@ -2048,6 +2055,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>  
>  	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
>  	if (fd < 0) {
> +		file->private_data = NULL;
>  		fput(file);
>  		goto out;
>  	}
> 

is fundamentally wrong; you really shouldn't take over the cleanups
if you ever do fput().