Re: [syzbot] [net?] INFO: rcu detected stall in unix_release

[Jamal Hadi Salim]

On Thu, Aug 17, 2023 at 12:30 PM Jamal Hadi Salim <jhs@xxxxxxxxxxxx> wrote:
>
> On Wed, Aug 16, 2023 at 6:58 PM Vladimir Oltean <vladimir.oltean@xxxxxxx> wrote:
> >
> > Hi Jakub,
> >
> > On Tue, Aug 15, 2023 at 02:28:21PM +0300, Vladimir Oltean wrote:
> > > On Mon, Aug 14, 2023 at 04:03:03PM -0700, Jakub Kicinski wrote:
> > > > Hi Vladimir, any ideas for this one?
> > > > The bisection looks pooped, FWIW, looks like a taprio inf loop.
> > >
> > > I'm looking into it.
> >
> > Here's what I've found out and what help I'll need going forward.
> >
> > Indeed there is an infinite loop in taprio_dequeue() -> taprio_dequeue_tc_priority(),
> > leading to an RCU stall.
> >
> > Short description of taprio_dequeue_tc_priority(): it cycles
> > q->cur_txq[tc] in the range between [ offset, offset + count ), where:
> >
> >         int offset = dev->tc_to_txq[tc].offset;
> >         int count = dev->tc_to_txq[tc].count;
> >
> > with the initial q->cur_txq[tc], aka the "first_txq" variable, being set
> > by the control path: taprio_change(), also called by taprio_init():
> >
> >         if (mqprio) {
> >                 (...)
> >                 for (i = 0; i < mqprio->num_tc; i++) {
> >                         (...)
> >                         q->cur_txq[i] = mqprio->offset[i];
> >                 }
> >         }
> >
> > In the buggy case that leads to the RCU stall, the line in taprio_change()
> > which sets q->cur_txq[i] never gets executed. So first_txq will be 0
> > (pre-initialized memory), and if that's outside of the [ offset, offset + count )
> > range that taprio_dequeue_tc_priority() -> taprio_next_tc_txq() expects
> > to cycle through, the kernel is toast.
> >
> > The nitty gritty of that is boring. What's not boring is how come the
> > control path skips the q->cur_txq[i] assignment. It's because "mqprio"
> > is NULL, and that's because taprio_change() (important: also tail-called
> > from taprio_init()) has this logic to detect a change in the traffic
> > class settings of the device, compared to the passed TCA_TAPRIO_ATTR_PRIOMAP
> > netlink attribute:
> >
> >         /* no changes - no new mqprio settings */
> >         if (!taprio_mqprio_cmp(q, dev, mqprio))
> >                 mqprio = NULL;
> >
> > And what happens is that:
> > - we go through taprio_init()
> > - a TCA_TAPRIO_ATTR_PRIOMAP gets passed to us
> > - taprio_mqprio_cmp() sees that there's no change compared to the
> >   netdev's existing traffic class config
> > - taprio_change() sets "mqprio" to NULL, ignoring the given
> >   TCA_TAPRIO_ATTR_PRIOMAP
> > - we skip modifying q->cur_txq[i], as if it was a taprio_change() call
> >   that came straight from Qdisc_ops :: change(), rather than what it
> >   really is: one from Qdisc_ops :: init()
> >
> > So the next question: why does taprio_mqprio_cmp() see that there's no
> > change? Because there is no change. When Qdisc_ops :: init() is called,
> > the netdev really has a non-zero dev->num_tc, prio_tc_map, tc_to_txq and
> > all that.
> >
> > But why? A previous taprio, if that existed, will call taprio_destroy()
> > -> netdev_reset_tc(), so it won't leave state behind that will hinder
> > the current taprio. Checking for stuff in the netdev state is just so
> > that taprio_change() can distinguish between a direct Qdisc_ops :: change()
> > call vs one coming from init().
> >
> > Finally, here's where the syzbot repro becomes relevant. It crafts the
> > RTM_NEWQDISC netlink message in such a way, that it makes tc_modify_qdisc()
> > in sch_api.c call a Qdisc_ops sequence with which taprio wasn't written
> > in mind.
> >
> > With "tc qdisc replace && tc qdisc replace", tc_modify_qdisc() is
> > supposed to call init() the first time and replace() the second time.
> > What the repro does is make the above sequence call two init() methods
> > back to back.
> >
> > To create an iproute2-based reproducer rather than the C one provided by
> > syzbot, we need this iproute2 change:
> >
> > diff --git a/tc/tc_qdisc.c b/tc/tc_qdisc.c
> > index 56086c43b7fa..20d9622b6bf3 100644
> > --- a/tc/tc_qdisc.c
> > +++ b/tc/tc_qdisc.c
> > @@ -448,6 +448,8 @@ int do_qdisc(int argc, char **argv)
> >                 return tc_qdisc_modify(RTM_NEWQDISC, NLM_F_EXCL|NLM_F_CREATE, argc-1, argv+1);
> >         if (matches(*argv, "change") == 0)
> >                 return tc_qdisc_modify(RTM_NEWQDISC, 0, argc-1, argv+1);
> > +       if (strcmp(*argv, "replace-exclusive") == 0)
> > +               return tc_qdisc_modify(RTM_NEWQDISC, NLM_F_CREATE|NLM_F_REPLACE|NLM_F_EXCL, argc-1, argv+1);
> >         if (matches(*argv, "replace") == 0)
> >                 return tc_qdisc_modify(RTM_NEWQDISC, NLM_F_CREATE|NLM_F_REPLACE, argc-1, argv+1);
> >         if (matches(*argv, "link") == 0)
> >
> > which basically implements a crafted alternative of "tc qdisc replace"
> > which also sets the NLM_F_EXCL flag in n->nlmsg_flags.
> >
> > Then, the minimal repro script can simply be expressed as:
> >
> > #!/bin/bash
> >
> > ip link add veth0 numtxqueues 16 numrxqueues 16 type veth peer name veth1
> > ip link set veth0 up && ip link set veth1 up
> >
> > for ((i = 0; i < 2; i++)); do
> >         tc qdisc replace-exclusive dev veth0 root stab overhead 24 taprio \
> >                 num_tc 2 map 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 \
> >                 queues 8@0 4@8 \
> >                 clockid REALTIME \
> >                 base-time 0 \
> >                 cycle-time 61679 \
> >                 sched-entry S 0 54336 \
> >                 sched-entry S 0x8a27 7343 \
> >                 max-sdu 18343 18343 \
> >                 flags 0
> > done
> >
> > ip link del veth0
> >
> > Here's how things go sideways if sch_api.c goes through the Qdisc_ops :: init()
> > code path instead of change() for the second Qdisc.
> >
> > The first taprio_attach() (i=0) will attach the root taprio Qdisc (aka itself)
> > to all netdev TX queues, and qdisc_put() the existing pfifo default Qdiscs.
> >
> > When the taprio_init() method executes for i=1, taprio_destroy() hasn't
> > been called yet. So neither has netdev_reset_tc() been called, and
> > that's part of the problem (the one that causes the infinite loop in
> > dequeue()).
> >
> > But, taprio_destroy() will finally get called for the initial taprio
> > created at i=0. The call trace looks like this:
> >
> >  rtnetlink_rcv_msg()
> >  -> tc_modify_qdisc()
> >     -> qdisc_graft()
> >        -> taprio_attach() for i=1
> >           -> qdisc_put() for the old Qdiscs attached to the TX queues, aka the taprio from i=0
> >              -> __qdisc_destroy()
> >                 -> taprio_destroy()
> >
> > What's more interesting is that the late taprio_destroy() for i=0
> > effectively destroys the netdev state - the netdev_reset_tc() call -
> > done by taprio_init() -> taprio_change() for i=1, and that can't be
> > too good, either. Even if there's no immediately observable hang, the
> > traffic classes are reset even though the Qdisc thinks they aren't.
> >
> > Taprio isn't the only one affected by this. Mqprio also has the pattern
> > of calling netdev_set_num_tc() from Qdisc_ops :: init() and destroy().
> > But with the possibility of destroy(i=0) not being serialized with
> > init(i=1), that's buggy.
> >
> > Sorry for the long message. This is where I'm at. For me, this is the
> > bottom of where things are intuitive. I don't understand what is
> > considered to be expected behavior from tc_modify_qdisc(), and what is
> > considered to be sane Qdisc-facing API, and I need help.
> >
> > I've completely stopped debugging when I saw that the code enters
> > through this path at i=1, so I really can't tell you more:
> >
> >                                 /* This magic test requires explanation.
> >                                  *
> >                                  *   We know, that some child q is already
> >                                  *   attached to this parent and have choice:
> >                                  *   either to change it or to create/graft new one.
> >                                  *
> >                                  *   1. We are allowed to create/graft only
> >                                  *   if CREATE and REPLACE flags are set.
> >                                  *
> >                                  *   2. If EXCL is set, requestor wanted to say,
> >                                  *   that qdisc tcm_handle is not expected
> >                                  *   to exist, so that we choose create/graft too.
> >                                  *
> >                                  *   3. The last case is when no flags are set.
> >                                  *   Alas, it is sort of hole in API, we
> >                                  *   cannot decide what to do unambiguously.
> >                                  *   For now we select create/graft, if
> >                                  *   user gave KIND, which does not match existing.
> >                                  */
> >                                 if ((n->nlmsg_flags & NLM_F_CREATE) &&
> >                                     (n->nlmsg_flags & NLM_F_REPLACE) &&
> >                                     ((n->nlmsg_flags & NLM_F_EXCL) ||
> >                                      (tca[TCA_KIND] &&
> >                                       nla_strcmp(tca[TCA_KIND], q->ops->id)))) {
> >                                         netdev_err(dev, "magic test\n");
> >                                         goto create_n_graft;
> >                                 }
> >
> > I've added more Qdisc people to the discussion. The problem description
> > is pretty much self-contained in this email, and going to the original
> > syzbot report won't bring much else.
> >
>
> I will take a look tommorow.
>

Can you try the attached patchlet?

cheers,
jamal

> cheers,
> jamal
>
> > There are multiple workarounds that can be done in taprio (and mqprio)
> > depending on what is considered as being sane API. Though I don't want
> > to get ahead of myself. Maybe there is a way to fast-forward the
> > qdisc_destroy() of the previous taprio so it doesn't overlap with the
> > new one's qdisc_create().

Attachment: patchlet-qdisc
Description: Binary data