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

From: Al Viro <[email-masked]>
Date: Thu, 7 Jun 2018 22:26:31 +0100

On Thu, Jun 07, 2018 at 01:39:49PM -0700, Cong Wang wrote:
> fchownat() doesn't even hold refcnt of fd until it figures out
> fd is really needed (otherwise is ignored) and releases it after
> it resolves the path. This means sock_close() could race with
> sockfs_setattr(), which leads to a NULL pointer dereference
> since typically we set sock->sk to NULL in ->release().
> 
> As pointed out by Al, this is unique to sockfs. So we can fix this
> in socket layer by acquiring inode_lock in sock_close() and
> checking against NULL in sockfs_setattr().

That looks like a massive overkill - it's way heavier than it should be.
And it's very likely to trigger shitloads of deadlock warnings, some
possibly even true.