diff mbox series

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

Message ID 20210302195639.12604-2-tim.gardner@canonical.com
State New
Headers show
Series CVE-2020-0466 | expand

Commit Message

Tim Gardner March 2, 2021, 7:56 p.m. UTC
From: Marc Zyngier <maz@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@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(back ported from commit b158e91610c76c5d9c61c4e990d56405b62bf05a linux-5.4.y)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
---
 fs/eventpoll.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Comments

Thadeu Lima de Souza Cascardo March 4, 2021, 11:05 a.m. UTC | #1
On Tue, Mar 02, 2021 at 12:56:37PM -0700, Tim Gardner wrote:
> From: Marc Zyngier <maz@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@vger.kernel.org
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> (back ported from commit b158e91610c76c5d9c61c4e990d56405b62bf05a linux-5.4.y)
> Signed-off-by: Tim Gardner <tim.gardner@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
> 
> 
> -- 
> kernel-team mailing list
> kernel-team@lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team
diff mbox series

Patch

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: