Date: Tue, 28 Jun 2016 15:51:57 -0700 From: "K. Macy" <kmacy@freebsd.org> To: Matthew Macy <mmacy@nextbsd.org> Cc: Julien Charbon <jch@freebsd.org>, Randall Stewart <rrs@netflix.com>, "current@freebsd.org" <current@freebsd.org>, Hans Petter Selasky <hselasky@freebsd.org>, "freebsd-net@freebsd.org" <net@freebsd.org> Subject: Re: panic with tcp timers Message-ID: <CAHM0Q_OEYhAqqgPj3Zn9z3KNn7c-=B8CLFWR=S7_fU2ePknt7A@mail.gmail.com> In-Reply-To: <15598235139.12175f84421756.2471769249719458878@nextbsd.org> References: <20160617045319.GE1076@FreeBSD.org> <1f28844b-b4ea-b544-3892-811f2be327b9@freebsd.org> <20160620073917.GI1076@FreeBSD.org> <1d18d0e2-3e42-cb26-928c-2989d0751884@freebsd.org> <20160620095822.GJ1076@FreeBSD.org> <74bb31b7-a9f5-3d0c-eea0-681872e6f09b@freebsd.org> <18D94615-810E-4E79-A889-4B0CC70F9E45@netflix.com> <6E52CA6A-2153-4EF9-A3E1-97CB0D07EB28@freebsd.org> <ae21858a-e162-6aad-1597-eeff614624c9@freebsd.org> <15598235139.12175f84421756.2471769249719458878@nextbsd.org>
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On Tue, Jun 28, 2016 at 10:51 AM, Matthew Macy <mmacy@nextbsd.org> wrote:
> You guys should really look at Samy Bahra's epoch based reclamation. I solved a similar problem in drm/linuxkpi using it.
The point being that this is a bug in the TCP life cycle handling
_not_ in callouts. Churning the callout interface is not the best /
only solution.
-M
>
> ---- On Tue, 28 Jun 2016 02:58:56 -0700 Julien Charbon <jch@freebsd.org> wrote ----
> >
> > Hi Randall,
> >
> > On 6/25/16 4:41 PM, Randall Stewart via freebsd-net wrote:
> > > Ok
> > >
> > > Lets try this again with my source changed to my @freebsd.net :-)
> > >
> > > Now I am also attaching a patch for you Gleb, this will take some poking to
> > > get in to your NF-head since it incorporates some changes we made earlier.
> > >
> > > I think this will fix the problem.. i.e. dealing with two locks in the callout system (which it was
> > > never meant to have done)..
> > >
> > > Note we probably can move the code to use the callout lock init now.. but lets see if this works
> > > on your setup on c096 and if so we can think about doing that.
> >
> > Thanks for proposing a patch. I believe your patch will work with
> > callout lock init, but not without: You still have a use-after-free
> > issue on the tcpcb without callout lock init.
> >
> > The case being subtle as usual, let me try to describe that could happen:
> >
> > With your patch we have:
> >
> > void
> > tcp_timer_keep(void *xtp)
> > {
> > struct tcpcb *tp = xtp;
> > struct tcptemp *t_template;
> > struct inpcb *inp;
> > CURVNET_SET(tp->t_vnet);
> > #ifdef TCPDEBUG
> > int ostate;
> >
> > ostate = tp->t_state;
> > #endif
> > inp = tp->t_inpcb;
> > KASSERT(inp != NULL, ("%s: tp %p tp->t_inpcb == NULL", __func__,
> > tp));
> > INP_WLOCK(inp);
> > if (callout_pending(&tp->t_timers->tt_keep) ### Use after free
> > of tp here
> > !callout_active(&tp->t_timers->tt_keep)) {
> > INP_WUNLOCK(inp);
> > CURVNET_RESTORE();
> > return;
> > }
> > ...
> >
> > The use-after-free scenario:
> >
> > [CPU 1] the callout fires, tcp_timer_keep entered
> > [CPU 1] blocks on INP_WLOCK(inp);
> > [CPU 2] schedules tcp_timer_keep with callout_reset()
> > [CPU 2] tcp_discardcb called
> > [CPU 2] tcp_timer_keep callout successfully canceled
> > [CPU 2] tcpcb freed
> > [CPU 1] unblocks, the tcpcb is used
> >
> > Then the tcpcb will used just after being freed... Might also crash or
> > not depending in the case.
> >
> > Extra notes:
> >
> > o The invariant I see here is: The "callout successfully canceled"
> > step should never happen when "the callout is currently being executed".
> >
> > o Solutions I see to enforce this invariant:
> >
> > - First solution: Use callout lock init with inp lock, your patch
> > seems to permit that now.
> >
> > - Second solution: Change callout_async_drain() behavior: It can
> > return 0 (fail) when the callout is currently being executed (no matter
> > what).
> >
> > - Third solution: Don't trust callout_async_drain(callout) return
> > value of 1 (success) if the previous call of callout_reset(callout)
> > returned 0 (fail). That was the exact purpose of r284261 change, but
> > this solution is also step backward in modernization of TCP
> > timers/callout...
> >
> > https://svnweb.freebsd.org/base/stable/10/sys/netinet/tcp_timer.c?r1=284261&r2=284260&pathrev=284261
> >
> > Hopefully my description is clear enough...
> >
> > --
> > Julien
> >
> >
>
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