---- On Tue, 28 Jun 2016 15:51:57 -0700 K. Macy <km...@freebsd.org> wrote ----
> On Tue, Jun 28, 2016 at 10:51 AM, Matthew Macy <mm...@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
Please see see D7017/D7018 as an example for an ultimately more robust solution
than continued fiddling with the callout interface.
https://reviews.freebsd.org/D7018
Cheers.
-M
> > ---- On Tue, 28 Jun 2016 02:58:56 -0700 Julien Charbon <j...@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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