Re: KASAN: use-after-free Read in userfaultfd_release (2)

[Al Viro]

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().