Re: [PATCH net 2/2] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

[Jamal Hadi Salim]

On Sat, Jun 10, 2023 at 11:30 PM Peilin Ye <yepeilin.cs@xxxxxxxxx> wrote:
>
> From: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>
>
> mini_Qdisc_pair::p_miniq is a double pointer to mini_Qdisc, initialized
> in ingress_init() to point to net_device::miniq_ingress.  ingress Qdiscs
> access this per-net_device pointer in mini_qdisc_pair_swap().  Similar
> for clsact Qdiscs and miniq_egress.
>
> Unfortunately, after introducing RTNL-unlocked RTM_{NEW,DEL,GET}TFILTER
> requests (thanks Hillf Danton for the hint), when replacing ingress or
> clsact Qdiscs, for example, the old Qdisc ("@old") could access the same
> miniq_{in,e}gress pointer(s) concurrently with the new Qdisc ("@new"),
> causing race conditions [1] including a use-after-free bug in
> mini_qdisc_pair_swap() reported by syzbot:
>
>  BUG: KASAN: slab-use-after-free in mini_qdisc_pair_swap+0x1c2/0x1f0 net/sched/sch_generic.c:1573
>  Write of size 8 at addr ffff888045b31308 by task syz-executor690/14901
> ...
>  Call Trace:
>   <TASK>
>   __dump_stack lib/dump_stack.c:88 [inline]
>   dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106
>   print_address_description.constprop.0+0x2c/0x3c0 mm/kasan/report.c:319
>   print_report mm/kasan/report.c:430 [inline]
>   kasan_report+0x11c/0x130 mm/kasan/report.c:536
>   mini_qdisc_pair_swap+0x1c2/0x1f0 net/sched/sch_generic.c:1573
>   tcf_chain_head_change_item net/sched/cls_api.c:495 [inline]
>   tcf_chain0_head_change.isra.0+0xb9/0x120 net/sched/cls_api.c:509
>   tcf_chain_tp_insert net/sched/cls_api.c:1826 [inline]
>   tcf_chain_tp_insert_unique net/sched/cls_api.c:1875 [inline]
>   tc_new_tfilter+0x1de6/0x2290 net/sched/cls_api.c:2266
> ...
>
> @old and @new should not affect each other.  In other words, @old should
> never modify miniq_{in,e}gress after @new, and @new should not update
> @old's RCU state.
>
> Fixing without changing sch_api.c turned out to be difficult (please
> refer to Closes: for discussions).  Instead, make sure @new's first call
> always happen after @old's last call (in {ingress,clsact}_destroy()) has
> finished:
>
> In qdisc_graft(), return -EBUSY if @old has any ongoing filter requests,
> and call qdisc_destroy() for @old before grafting @new.
>
> Introduce qdisc_refcount_dec_if_one() as the counterpart of
> qdisc_refcount_inc_nz() used for filter requests.  Introduce a
> non-static version of qdisc_destroy() that does a TCQ_F_BUILTIN check,
> just like qdisc_put() etc.
>
> Depends on patch "net/sched: Refactor qdisc_graft() for ingress and
> clsact Qdiscs".
>
> [1] To illustrate, the syzkaller reproducer adds ingress Qdiscs under
> TC_H_ROOT (no longer possible after commit c7cfbd115001 ("net/sched:
> sch_ingress: Only create under TC_H_INGRESS")) on eth0 that has 8
> transmission queues:
>
>   Thread 1 creates ingress Qdisc A (containing mini Qdisc a1 and a2),
>   then adds a flower filter X to A.
>
>   Thread 2 creates another ingress Qdisc B (containing mini Qdisc b1 and
>   b2) to replace A, then adds a flower filter Y to B.
>
>  Thread 1               A's refcnt   Thread 2
>   RTM_NEWQDISC (A, RTNL-locked)
>    qdisc_create(A)               1
>    qdisc_graft(A)                9
>
>   RTM_NEWTFILTER (X, RTNL-unlocked)
>    __tcf_qdisc_find(A)          10
>    tcf_chain0_head_change(A)
>    mini_qdisc_pair_swap(A) (1st)
>             |
>             |                         RTM_NEWQDISC (B, RTNL-locked)
>          RCU sync                2     qdisc_graft(B)
>             |                    1     notify_and_destroy(A)
>             |
>    tcf_block_release(A)          0    RTM_NEWTFILTER (Y, RTNL-unlocked)
>    qdisc_destroy(A)                    tcf_chain0_head_change(B)
>    tcf_chain0_head_change_cb_del(A)    mini_qdisc_pair_swap(B) (2nd)
>    mini_qdisc_pair_swap(A) (3rd)                |
>            ...                                 ...
>
> Here, B calls mini_qdisc_pair_swap(), pointing eth0->miniq_ingress to
> its mini Qdisc, b1.  Then, A calls mini_qdisc_pair_swap() again during
> ingress_destroy(), setting eth0->miniq_ingress to NULL, so ingress
> packets on eth0 will not find filter Y in sch_handle_ingress().
>
> This is just one of the possible consequences of concurrently accessing
> miniq_{in,e}gress pointers.
>
> Fixes: 7a096d579e8e ("net: sched: ingress: set 'unlocked' flag for Qdisc ops")
> Fixes: 87f373921c4e ("net: sched: ingress: set 'unlocked' flag for clsact Qdisc ops")
> Reported-by: syzbot+b53a9c0d1ea4ad62da8b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/r/0000000000006cf87705f79acf1a@xxxxxxxxxx/
> Cc: Hillf Danton <hdanton@xxxxxxxx>
> Cc: Vlad Buslov <vladbu@xxxxxxxxxxxx>
> Signed-off-by: Peilin Ye <peilin.ye@xxxxxxxxxxxxx>

Acked-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>

cheers,
jamal

> ---
>  include/net/sch_generic.h |  8 ++++++++
>  net/sched/sch_api.c       | 28 +++++++++++++++++++++++-----
>  net/sched/sch_generic.c   | 14 +++++++++++---
>  3 files changed, 42 insertions(+), 8 deletions(-)
>
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index 27271f2b37cb..12eadecf8cd0 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -137,6 +137,13 @@ static inline void qdisc_refcount_inc(struct Qdisc *qdisc)
>         refcount_inc(&qdisc->refcnt);
>  }
>
> +static inline bool qdisc_refcount_dec_if_one(struct Qdisc *qdisc)
> +{
> +       if (qdisc->flags & TCQ_F_BUILTIN)
> +               return true;
> +       return refcount_dec_if_one(&qdisc->refcnt);
> +}
> +
>  /* Intended to be used by unlocked users, when concurrent qdisc release is
>   * possible.
>   */
> @@ -652,6 +659,7 @@ void dev_deactivate_many(struct list_head *head);
>  struct Qdisc *dev_graft_qdisc(struct netdev_queue *dev_queue,
>                               struct Qdisc *qdisc);
>  void qdisc_reset(struct Qdisc *qdisc);
> +void qdisc_destroy(struct Qdisc *qdisc);
>  void qdisc_put(struct Qdisc *qdisc);
>  void qdisc_put_unlocked(struct Qdisc *qdisc);
>  void qdisc_tree_reduce_backlog(struct Qdisc *qdisc, int n, int len);
> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
> index 094ca3a5b633..aa6b1fe65151 100644
> --- a/net/sched/sch_api.c
> +++ b/net/sched/sch_api.c
> @@ -1086,10 +1086,22 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,
>                 if ((q && q->flags & TCQ_F_INGRESS) ||
>                     (new && new->flags & TCQ_F_INGRESS)) {
>                         ingress = 1;
> -                       if (!dev_ingress_queue(dev)) {
> +                       dev_queue = dev_ingress_queue(dev);
> +                       if (!dev_queue) {
>                                 NL_SET_ERR_MSG(extack, "Device does not have an ingress queue");
>                                 return -ENOENT;
>                         }
> +
> +                       q = rtnl_dereference(dev_queue->qdisc_sleeping);
> +
> +                       /* This is the counterpart of that qdisc_refcount_inc_nz() call in
> +                        * __tcf_qdisc_find() for filter requests.
> +                        */
> +                       if (!qdisc_refcount_dec_if_one(q)) {
> +                               NL_SET_ERR_MSG(extack,
> +                                              "Current ingress or clsact Qdisc has ongoing filter requests");
> +                               return -EBUSY;
> +                       }
>                 }
>
>                 if (dev->flags & IFF_UP)
> @@ -1110,8 +1122,16 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,
>                                 qdisc_put(old);
>                         }
>                 } else {
> -                       dev_queue = dev_ingress_queue(dev);
> -                       old = dev_graft_qdisc(dev_queue, new);
> +                       old = dev_graft_qdisc(dev_queue, NULL);
> +
> +                       /* {ingress,clsact}_destroy() @old before grafting @new to avoid
> +                        * unprotected concurrent accesses to net_device::miniq_{in,e}gress
> +                        * pointer(s) in mini_qdisc_pair_swap().
> +                        */
> +                       qdisc_notify(net, skb, n, classid, old, new, extack);
> +                       qdisc_destroy(old);
> +
> +                       dev_graft_qdisc(dev_queue, new);
>                 }
>
>  skip:
> @@ -1125,8 +1145,6 @@ static int qdisc_graft(struct net_device *dev, struct Qdisc *parent,
>
>                         if (new && new->ops->attach)
>                                 new->ops->attach(new);
> -               } else {
> -                       notify_and_destroy(net, skb, n, classid, old, new, extack);
>                 }
>
>                 if (dev->flags & IFF_UP)
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 3248259eba32..5d7e23f4cc0e 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -1046,7 +1046,7 @@ static void qdisc_free_cb(struct rcu_head *head)
>         qdisc_free(q);
>  }
>
> -static void qdisc_destroy(struct Qdisc *qdisc)
> +static void __qdisc_destroy(struct Qdisc *qdisc)
>  {
>         const struct Qdisc_ops  *ops = qdisc->ops;
>
> @@ -1070,6 +1070,14 @@ static void qdisc_destroy(struct Qdisc *qdisc)
>         call_rcu(&qdisc->rcu, qdisc_free_cb);
>  }
>
> +void qdisc_destroy(struct Qdisc *qdisc)
> +{
> +       if (qdisc->flags & TCQ_F_BUILTIN)
> +               return;
> +
> +       __qdisc_destroy(qdisc);
> +}
> +
>  void qdisc_put(struct Qdisc *qdisc)
>  {
>         if (!qdisc)
> @@ -1079,7 +1087,7 @@ void qdisc_put(struct Qdisc *qdisc)
>             !refcount_dec_and_test(&qdisc->refcnt))
>                 return;
>
> -       qdisc_destroy(qdisc);
> +       __qdisc_destroy(qdisc);
>  }
>  EXPORT_SYMBOL(qdisc_put);
>
> @@ -1094,7 +1102,7 @@ void qdisc_put_unlocked(struct Qdisc *qdisc)
>             !refcount_dec_and_rtnl_lock(&qdisc->refcnt))
>                 return;
>
> -       qdisc_destroy(qdisc);
> +       __qdisc_destroy(qdisc);
>         rtnl_unlock();
>  }
>  EXPORT_SYMBOL(qdisc_put_unlocked);
> --
> 2.20.1
>