linux-kernel - [BUG] fs/notify/mark: A potential use after free in fsnotify_put_mark

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Message-ID: <39095113.1936a.178781a774a.Coremail.lyl2019@mail.ustc.edu.cn>
Date:   Sun, 28 Mar 2021 17:11:43 +0800 (GMT+08:00)
From:   lyl2019@...l.ustc.edu.cn
To:     jack@...e.cz, amir73il@...il.com
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [BUG] fs/notify/mark: A potential use after free in
 fsnotify_put_mark_wake

Hi,
    My static analyzer tool reported a use after free in fsnotify_put_mark_wake
of the file: fs/notify/mark.c.

In fsnotify_put_mark_wake, it calls fsnotify_put_mark(mark). Inside the function
fsnotify_put_mark(), if conn is NULL, it will call fsnotify_final_mark_destroy(mark)
to free mark->group by fsnotify_put_group(group) and return. I also had inspected
the implementation of fsnotify_put_group() and found that there is no cleanup operation
about group->user_waits.

But after fsnotify_put_mark_wake() returned, mark->group is still used by 
if (atomic_dec_and_test(&group->user_waits) && group->shutdown) and later.

Is this an issue?

Thanks.