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

[Cong Wang <xiyou.wangcong@gmail.com>]

On Thu, Jun 7, 2018 at 3:04 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Thu, Jun 07, 2018 at 02:45:58PM -0700, Cong Wang wrote:
>> On Thu, Jun 7, 2018 at 2:26 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>> > 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.
>>
>> I don't see any other quick way to fix this. My initial thought is
>> to keep that refcnt until path_put(), apparently you don't like it
>> either.
>
> 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.