[PATCH 5.10 20/25] fanotify: Fix stale file descriptor in copy_event_to_user()

Greg Kroah-Hartman
2022-02-04T09:22:24Z

From: Dan Carpenter <dan.carpenter@oracle.com>

commit ee12595147ac1fbfb5bcb23837e26dd58d94b15d upstream.

This code calls fd_install() which gives the userspace access to the fd.
Then if copy_info_records_to_user() fails it calls put_unused_fd(fd) but
that will not release it and leads to a stale entry in the file
descriptor table.

Generally you can't trust the fd after a call to fd_install(). The fix
is to delay the fd_install() until everything else has succeeded.

Fortunately it requires CAP_SYS_ADMIN to reach this code so the security
impact is less.

Fixes: f644bc449b37 ("fanotify: fix copy_event_to_user() fid error clean up")
Link: https://lore.kernel.org/r/20220128195656.GA26981@kili
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mathias Krause <minipli@grsecurity.net>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/notify/fanotify/fanotify_user.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -366,9 +366,6 @@ static ssize_t copy_event_to_user(struct
if (fanotify_is_perm_event(event->mask))
FANOTIFY_PERM(event)->fd = fd;

- if (f)
- fd_install(fd, f);
-
/* Event info records order is: dir fid + name, child fid */
if (fanotify_event_dir_fh_len(event)) {
info_type = info->name_len ? FAN_EVENT_INFO_TYPE_DFID_NAME :
@@ -432,6 +429,9 @@ static ssize_t copy_event_to_user(struct
count -= ret;
}

+ if (f)
+ fd_install(fd, f);
+
return metadata.event_len;