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

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Date:   Mon, 29 Mar 2021 15:57:40 +0200
From:   Jan Kara <jack@...e.cz>
To:     lyl2019@...l.ustc.edu.cn
Cc:     jack@...e.cz, amir73il@...il.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [BUG] fs/notify/mark: A potential use after free in
 fsnotify_put_mark_wake

Hello!


On Sun 28-03-21 17:11:43, lyl2019@...l.ustc.edu.cn wrote:
>     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?

I don't think this scenario is possible. fsnotify_put_mark_wake() can be
called only for marks attached to objects and these have mark->conn !=
NULL and we are sure that fsnotify_destroy_group() will wait for all such
marks to be torn down and freed before dropping last group reference and
freeing the group.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR