From: Al Viro <viro@ZenIV•linux.org.uk>
To: Cong Wang <xiyou.wangcong@gmail•com>
Cc: Linux Kernel Network Developers <netdev@vger•kernel.org>,
shankarapailoor <shankarapailoor@gmail•com>,
Tetsuo Handa <penguin-kernel@i-love•sakura.ne.jp>,
Lorenzo Colitti <lorenzo@google•com>
Subject: Re: [Patch net] socket: close race condition between sock_close() and sockfs_setattr()
Date: Thu, 7 Jun 2018 23:32:43 +0100 [thread overview]
Message-ID: <20180607223242.GY30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAM_iQpWs8pLANoQhUBsJLfwJc6V5db69v1SFeLg_Uu0LdgJzMA@mail.gmail.com>
On Thu, Jun 07, 2018 at 03:15:15PM -0700, Cong Wang wrote:
> > You do realize that the same ->setattr() can be called by way of
> > chown() on /proc/self/fd/<n>, right? What would you do there -
> > bump refcount on that struct file when traversing that symlink and
> > hold it past the end of pathname resolution, until... what exactly?
>
> I was thinking about this:
>
> error = user_path_at(dfd,....); // hold dfd when needed
>
> if (!error) {
> error = chown_common(&path, mode);
> path_put(&path); // release dfd if held
>
> With this, we can guarantee ->release() is only possibly called
> after chown_common() which is after ->setattr() too.
No, we can't. You are assuming that there *is* dfd and that it points
to the opened socket we are going to operate upon. That is not guaranteed.
At all. If e.g. 42 is a file descriptor of an opened socket, plain chown(2)
on /proc/self/fd/42 will trigger that ->setattr(). No dfd in sight.
We do run across an opened file at some point, all right - when we traverse
the symlink in procfs. You can't bump ->f_count there. Even leaving aside
the "where would I stash the reference to that file?" and "how long would I
hold it?", you can't bump ->f_count on other process' files. That would
bugger the expectations of close(2) callers.
next prev parent reply other threads:[~2018-06-07 22:32 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-07 20:39 [Patch net] socket: close race condition between sock_close() and sockfs_setattr() Cong Wang
2018-06-07 21:26 ` Al Viro
2018-06-07 21:45 ` Cong Wang
2018-06-07 22:04 ` Al Viro
2018-06-07 22:15 ` Cong Wang
2018-06-07 22:32 ` Al Viro [this message]
2018-06-10 19:27 ` David Miller
2018-06-10 19:43 ` Cong Wang
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