From 6db723ca4098ec72cf1051fdbee470d281b1b026 Mon Sep 17 00:00:00 2001 From: Marc Zyngier Date: Wed, 19 Aug 2020 17:12:17 +0100 Subject: [PATCH] epoll: Keep a reference on files added to the check list 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 Signed-off-by: Al Viro Signed-off-by: Marc Zyngier Signed-off-by: Greg Kroah-Hartman Signed-off-by: Lee Jones Change-Id: I780923748a8387fa6b3174b952ca22994a377170 CVE-2020-0466 Signed-off-by: Kevin F. Haggerty --- fs/eventpoll.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index f2609ff320b..9797ab271b6 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1591,9 +1591,11 @@ 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); @@ -1637,6 +1639,7 @@ 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); } @@ -1773,8 +1776,10 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd, clear_tfile_check_list(); goto error_tgt_fput; } - } else + } else { + get_file(tfile); list_add(&tfile->f_tfile_llink, &tfile_check_list); + } } mutex_lock_nested(&ep->mtx, 0);