Re: [Patch net] socket: close race condition between sock_close() and sockfs_setattr()

[Al Viro <viro@ZenIV.linux.org.uk>]

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.