[PATCH 1/3] epoll: Keep a reference on files added to the check list
Tim Gardner
tim.gardner at canonical.com
Tue Mar 2 19:56:37 UTC 2021
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>
---
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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