netdev - Re: [Patch net] socket: close race condition between sock_close() and sockfs

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Date:   Thu, 7 Jun 2018 23:32:43 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        shankarapailoor <shankarapailoor@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: [Patch net] socket: close race condition between sock_close()
 and sockfs_setattr()

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.