Message-ID: <20150602134851.GA15847@hmsreliant.think-freely.org>
Date:	Tue, 2 Jun 2015 09:48:51 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Marcelo Ricardo Leitner <mleitner@...hat.com>
Cc:	netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Vlad Yasevich <vyasevich@...il.com>,
	Michio Honda <micchie@....wide.ad.jp>
Subject: Re: [PATCH] sctp: fix ASCONF list handling

On Mon, Jun 01, 2015 at 07:28:21PM -0300, Marcelo Ricardo Leitner wrote:
> On Mon, Jun 01, 2015 at 10:00:28AM -0400, Neil Horman wrote:
> > On Fri, May 29, 2015 at 01:50:37PM -0300, Marcelo Ricardo Leitner wrote:
> > > On Fri, May 29, 2015 at 09:17:26AM -0400, Neil Horman wrote:
> > > > On Thu, May 28, 2015 at 11:46:29AM -0300, Marcelo Ricardo Leitner wrote:
> > > > > On Thu, May 28, 2015 at 10:27:32AM -0300, Marcelo Ricardo Leitner wrote:
> > > > > > On Thu, May 28, 2015 at 08:17:27AM -0300, Marcelo Ricardo Leitner wrote:
> > > > > > > On Thu, May 28, 2015 at 06:15:11AM -0400, Neil Horman wrote:
> > > > > > > > On Wed, May 27, 2015 at 09:52:17PM -0300, mleitner@...hat.com wrote:
> > > > > > > > > From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > > > > > > >
> > > > > > > > > ->auto_asconf_splist is per namespace and mangled by functions like
> > > > > > > > > sctp_setsockopt_auto_asconf() which doesn't guarantee any serialization.
> > > > > > > > >
> > > > > > > > > Also, the call to inet_sk_copy_descendant() was backuping
> > > > > > > > > ->auto_asconf_list through the copy but was not honoring
> > > > > > > > > ->do_auto_asconf, which could lead to list corruption if it was
> > > > > > > > > different between both sockets.
> > > > > > > > >
> > > > > > > > > This commit thus fixes the list handling by adding a spinlock to protect
> > > > > > > > > against multiple writers and converts the list to be protected by RCU
> > > > > > > > > too, so that we don't have a lock inverstion issue at
> > > > > > > > > sctp_addr_wq_timeout_handler().
> > > > > > > > >
> > > > > > > > > And as this list now uses RCU, we cannot do such backup and restore
> > > > > > > > > while copying descendant data anymore as readers may be traversing the
> > > > > > > > > list meanwhile. We fix this by simply ignoring/not copying those fields,
> > > > > > > > > placed at the end of struct sctp_sock, so we can just ignore it together
> > > > > > > > > with struct ipv6_pinfo data. For that we create sctp_copy_descendant()
> > > > > > > > > so we don't clutter inet_sk_copy_descendant() with SCTP info.
> > > > > > > > >
> > > > > > > > > Issue was found with a test application that kept flipping sysctl
> > > > > > > > > default_auto_asconf on and off.
> > > > > > > > >
> > > > > > > > > Fixes: 9f7d653b67ae ("sctp: Add Auto-ASCONF support (core).")
> > > > > > > > > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > > > > > > > ---
> > > > > > > > >  include/net/netns/sctp.h   |  6 +++++-
> > > > > > > > >  include/net/sctp/structs.h |  2 ++
> > > > > > > > >  net/sctp/protocol.c        |  6 +++++-
> > > > > > > > >  net/sctp/socket.c          | 39 ++++++++++++++++++++++++++-------------
> > > > > > > > >  4 files changed, 38 insertions(+), 15 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/include/net/netns/sctp.h b/include/net/netns/sctp.h
> > > > > > > > > index 3573a81815ad9e0efb6ceb721eb066d3726419f0..e080bebb3147af39c8275261f57018eb01e917b0 100644
> > > > > > > > > --- a/include/net/netns/sctp.h
> > > > > > > > > +++ b/include/net/netns/sctp.h
> > > > > > > > > @@ -30,12 +30,15 @@ struct netns_sctp {
> > > > > > > > >  	struct list_head local_addr_list;
> > > > > > > > >  	struct list_head addr_waitq;
> > > > > > > > >  	struct timer_list addr_wq_timer;
> > > > > > > > > -	struct list_head auto_asconf_splist;
> > > > > > > > > +	struct list_head __rcu auto_asconf_splist;
> > > > > > > > You should use the addr_wq_lock here instead of creating a new lock, as thats
> > > > > > > > already used to protect most accesses to the list you are concerned about.
> > > > > > >
> > > > > > > Ok, that works too.
> > > > > > >
> > > > > > > > Though truthfully, that shouldn't be necessecary.  The list in question is only
> > > > > > > > read in one location and only written in one location.  You can likely just
> > > > > > > > rcu-ify, as the write side is in process context and protected by lock_sock.
> > > > > > >
> > > > > > > It should, it's not protected by lock_sock as this list resides in
> > > > > > > netns_sctp structure, which lock_sock doesn't cover. Write side is in
> > > > > > > process context yes, but this list is written in sctp_init_sock(),
> > > > > > > sctp_destroy_sock() and sctp_setsockopt_auto_asconf(), so one could
> > > > > > > trigger this by either creating/destroying sockets if
> > > > > > > default_auto_asconf=1 or just by creating a bunch of sockets and
> > > > > > > flipping asconf via setsockopt (or a combination of these operations).
> > > > > > > (I'll point this out in the changelog)
> > > > > >
> > > > > > Hmm.. by reusing addr_wq_lock we don't need to rcu-ify the list, as the
> > > > > > reader is inside that lock too, so I can just protect auto_asconf_splist
> > > > > > writers with addr_wq_lock.
> > > > > >
> > > > > > Nice, thanks Neil.
> > > > >
> > > > > Cannot really do that.. as that creates a lock inversion between
> > > > > sctp_destroy_sock() (which already holds lock_sock) and
> > > > > sctp_addr_wq_timeout_handler(), which first grabs addr_wq_lock and then
> > > > > locks socket by socket.
> > > > >
> > > > > Due to that, I'm afraid reusing this lock is not possible, and we should
> > > > > stick with the patch.. what do you think? (though I have to fix the nits
> > > > > in there)
> > > > >
> > > > I don't think thats accurate.  You are correct in that the the locks are taken
> > > > in opposing order, which would imply a lock inversion that could result in
> > > > deadlock, but we can avoid that by deferring the asconf list removal until after
> > > > sk_common_release and unlock_sock_bh is called in sctp_close.  That will make
> > > > the lock ordering consistent.  Alternatively, we can pre-emptively take the
> > > > asconf_lock in sctp_close before locking the socket.
> > > 
> > > For your first approach, deferring the asconf list removal, we can only
> > > do that reliably via some work queue, because we initialize asconf stuff
> > > on sctp_init_sock() and it should be de-initialized on its counterpart,
> > > sctp_destroy_sock(), as we have code like:
> > > 
> > > (same for ipv4)
> > > sctp_v6_create_accept_sk()
> > > {
> > > ...
> > >         if (newsk->sk_prot->init(newsk)) {
> > >                 sk_common_release(newsk);
> > >                 newsk = NULL;
> > >         }
> > > ...
> > > }
> > > 
> > > and at inet6_create() too:
> > >         if (sk->sk_prot->init) {
> > >                 err = sk->sk_prot->init(sk);
> > >                 if (err)
> > >                         sk_common_release(sk);
> > >         }
> > > 
> > > Or we (kind of) abuse of knowing that sctp_init_sock() cannot fail after
> > > initializing asconf and move asconf stuff from sctp_destroy_sock() to
> > > sctp_close(). AFAICT it could be enough, I'm just not a big fan of not
> > > having that similarity.
> > > 
> > > If we try to lock addr_wq_lock early in sctp_close(),
> > > sctp_destroy_sock() would be unprotected on above situations, but if we
> > > know that sctp_init_sock() won't fail after initilizating asconf, it
> > > wouldn't be a problem...
> > > 
> > > > I'd really rather avoid creating an additional lock here if we don't have to
> > > 
> > > I understand. I'm just not seeing another way out so far... I'll keep
> > > trying, but please I'm all ears to ideas ;)
> > > 
> > >   Marcelo
> > rcu-ify auto_asconf_splist, then just use the addr_wq_lock to protect the list
> > modification sites, as those are all handled at locations that already hold a
> > socket lock.  sctp_addr_wq_timeout_handler is a read-only function for
> > auto_asconf_splist, and so the spin_lock_bh there can be replaced with an
> > rcu_read_lock_bh call, breaking the lock inversion
> > Neil
> 
> That's still the lock inversion, because at
> sctp_addr_wq_timeout_handler() we are not grabbing addr_wq_lock just for
> this patch, it's already there and later inside that for_each it will
> lock socket by socket; while on sctp_destroy_sock() , it is called with
> socket lock already held and we would be trying to grab ->addr_wq_lock
> in order to protect the list change.
> 
I get this, thats why I suggested rcu-ifying the list, because those loops are
read-only sites, you can replace the spinlock with an rcu lock.

> We could rcu-ify addr_waitq too. Let sctp_addr_wq_timeout_handler() work
> on the list protected by rcu_read_lock_bh() and only at the bottom of
> that for() we grab addr_wq_lock and do the list change. Would require
> more changes than just that in order to work properly (and avoid
> double-frees through that timer and sctp_free_addr_wq() for example)
> 
Ah, sorry, yes I missed the list_del at the bottom.  You're correct though, if
you rcu-ify both lists, you can just lock the addr_wq_lock there in the proper
order and do the list modification under protection.  Double free protection
shouldn't be hard there, just add another list iterator that searches for an
element who's pointer is the same as the element you want to remove.  Only one
context should ever find it if its being removed.

Regards
Neil

>   Marcelo
> 
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