NACK: [PATCH 1/3] epoll: Keep a reference on files added to the check list

[Thadeu Lima de Souza Cascardo]

On Tue, Mar 02, 2021 at 12:56:37PM -0700, Tim Gardner wrote:
> From: Marc Zyngier <maz at kernel.org>
> 
> CVE-2020-0466
> 
> commit a9ed4a6560b8562b7e2e2bed9527e88001f7b682 upstream.
> 
> When adding a new fd to an epoll, and that this new fd is an
> epoll fd itself, we recursively scan the fds attached to it
> to detect cycles, and add non-epool files to a "check list"
> that gets subsequently parsed.
> 
> However, this check list isn't completely safe when deletions
> can happen concurrently. To sidestep the issue, make sure that
> a struct file placed on the check list sees its f_count increased,
> ensuring that a concurrent deletion won't result in the file
> disapearing from under our feet.
> 
> Cc: stable at vger.kernel.org
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> 
> (back ported from commit b158e91610c76c5d9c61c4e990d56405b62bf05a linux-5.4.y)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>

I managed to get a clean cherry-pick of upstream commit
9ed4a6560b8562b7e2e2bed9527e88001f7b682 ("epoll: Keep a reference on files
added to the check list").

Why haven't you tried that?

Cascardo.

> ---
>  fs/eventpoll.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index f5a481089893..1676b8b25074 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1972,9 +1972,11 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
>  			 * not already there, and calling reverse_path_check()
>  			 * during ep_insert().
>  			 */
> -			if (list_empty(&epi->ffd.file->f_tfile_llink))
> +			if (list_empty(&epi->ffd.file->f_tfile_llink)) {
> +				get_file(epi->ffd.file);
>  				list_add(&epi->ffd.file->f_tfile_llink,
>  					 &tfile_check_list);
> +			}
>  		}
>  	}
>  	mutex_unlock(&ep->mtx);
> @@ -2018,6 +2020,7 @@ static void clear_tfile_check_list(void)
>  		file = list_first_entry(&tfile_check_list, struct file,
>  					f_tfile_llink);
>  		list_del_init(&file->f_tfile_llink);
> +		fput(file);
>  	}
>  	INIT_LIST_HEAD(&tfile_check_list);
>  }
> @@ -2182,9 +2185,11 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
>  					clear_tfile_check_list();
>  					goto error_tgt_fput;
>  				}
> -			} else
> +			} else {
> +				get_file(tf.file);
>  				list_add(&tf.file->f_tfile_llink,
>  							&tfile_check_list);
> +			}
>  			error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
>  			if (error) {
>  out_del:
> -- 
> 2.17.1
> 
> 
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