fanotify: Fix stale file descriptor in copy_event_to_user()

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>

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 1026f67..2ff6bd8 100644 (file)
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -701,9 +701,6 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
        if (fanotify_is_perm_event(event->mask))
                FANOTIFY_PERM(event)->fd = fd;
 
-       if (f)
-               fd_install(fd, f);
-
        if (info_mode) {
                ret = copy_info_records_to_user(event, info, info_mode, pidfd,
                                                buf, count);
@@ -711,6 +708,9 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
                        goto out_close_fd;
        }
 
+       if (f)
+               fd_install(fd, f);
+
        return metadata.event_len;
 
 out_close_fd: