Re: fs, net: deadlock between bind/splice on af_unix

[Mateusz Guzik <mguzik@redhat.com>]

On Thu, Jan 26, 2017 at 09:11:07PM -0800, Cong Wang wrote:
> On Thu, Jan 26, 2017 at 3:29 PM, Mateusz Guzik <mguzik@redhat.com> wrote:
> > On Tue, Jan 17, 2017 at 01:21:48PM -0800, Cong Wang wrote:
> >> On Mon, Jan 16, 2017 at 1:32 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
> >> > On Fri, Dec 9, 2016 at 7:41 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >> >> On Thu, Dec 08, 2016 at 10:32:00PM -0800, Cong Wang wrote:
> >> >>
> >> >>> > Why do we do autobind there, anyway, and why is it conditional on
> >> >>> > SOCK_PASSCRED?  Note that e.g. for SOCK_STREAM we can bloody well get
> >> >>> > to sending stuff without autobind ever done - just use socketpair()
> >> >>> > to create that sucker and we won't be going through the connect()
> >> >>> > at all.
> >> >>>
> >> >>> In the case Dmitry reported, unix_dgram_sendmsg() calls unix_autobind(),
> >> >>> not SOCK_STREAM.
> >> >>
> >> >> Yes, I've noticed.  What I'm asking is what in there needs autobind triggered
> >> >> on sendmsg and why doesn't the same need affect the SOCK_STREAM case?
> >> >>
> >> >>> I guess some lock, perhaps the u->bindlock could be dropped before
> >> >>> acquiring the next one (sb_writer), but I need to double check.
> >> >>
> >> >> Bad idea, IMO - do you *want* autobind being able to come through while
> >> >> bind(2) is busy with mknod?
> >> >
> >> >
> >> > Ping. This is still happening on HEAD.
> >> >
> >>
> >> Thanks for your reminder. Mind to give the attached patch (compile only)
> >> a try? I take another approach to fix this deadlock, which moves the
> >> unix_mknod() out of unix->bindlock. Not sure if there is any unexpected
> >> impact with this way.
> >>
> >
> > I don't think this is the right approach.
> >
> > Currently the file creation is potponed until unix_bind can no longer
> > fail otherwise. With it reordered, it may be someone races you with a
> > different path and now you are left with a file to clean up. Except it
> > is quite unclear for me if you can unlink it.
> 
> What races do you mean here? If you mean someone could get a
> refcount of that file, it could happen no matter we have bindlock or not
> since it is visible once created. The filesystem layer should take care of
> the file refcount so all we need to do here is calling path_put() as in my
> patch. Or if you mean two threads calling unix_bind() could race without
> binlock, only one of them should succeed the other one just fails out.

Two threads can race and one fails with EINVAL.

With your patch there is a new file created and it is unclear what to
do with it - leaving it as it is sounds like the last resort and
unlinking it sounds extremely fishy as it opens you to games played by
the user.