Subject: Re: [PATCH v2 net 6/6] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

From: Vlad Buslov <[email-masked]>
Date: Tue, 23 May 2023 17:04:40 +0300

Hi Peilin,

Thanks again for the great analysis and investing effort into fixing
this!

On Mon 22 May 2023 at 16:55, Peilin Ye <[email-masked]> wrote:
> 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
> qdisc_destroy(), has finished:
>
> In qdisc_graft(), return -EAGAIN and tell the caller to replay
> (suggested by Vlad Buslov) if @old has any ongoing RTNL-unlocked 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 RTNL-unlocked 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 patch "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 only one of the possible consequences of concurrently accessing
> miniq_{in,e}gress pointers.  The point is clear though: again, A should
> never modify those per-net_device pointers after B, and B should not
> update A's RCU state.
>
> 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: [email-masked]
> Closes: https://lore.kernel.org/r/[email-masked]/
> Cc: Hillf Danton <[email-masked]>
> Cc: Vlad Buslov <[email-masked]>
> Signed-off-by: Peilin Ye <[email-masked]>
> ---
> changes in v2:
>   - replay the request if the current Qdisc has any ongoing RTNL-unlocked
>     filter requests (Vlad)
>   - minor changes in code comments and commit log
>
>  include/net/sch_generic.h |  8 ++++++++
>  net/sched/sch_api.c       | 32 ++++++++++++++++++++++++++------
>  net/sched/sch_generic.c   | 14 +++++++++++---
>  3 files changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index fab5ba3e61b7..3e9cc43cbc90 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 f72a581666a2..b3bafa6c1b44 100644
> --- a/net/sched/sch_api.c
> +++ b/net/sched/sch_api.c
> @@ -1080,10 +1080,18 @@ 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;
>  			}
> +
> +			/* Replay if the current ingress (or clsact) Qdisc has ongoing
> +			 * RTNL-unlocked filter request(s).  This is the counterpart of that
> +			 * qdisc_refcount_inc_nz() call in __tcf_qdisc_find().
> +			 */
> +			if (!qdisc_refcount_dec_if_one(dev_queue->qdisc_sleeping))
> +				return -EAGAIN;
>  		}
>  
>  		if (dev->flags & IFF_UP)
> @@ -1104,8 +1112,16 @@ 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:
> @@ -1119,8 +1135,6 @@ 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)
> @@ -1458,6 +1472,7 @@ static int tc_get_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
>  	struct Qdisc *p = NULL;
>  	int err;
>  
> +replay:

Perhaps also set q and p to NULL here? Even though on cursory look you
should get the same lookup result since the function is called under
rtnl_lock, tc_modify_qdisc() does this on replay ("Reinit, just in case
something touches this.") and tc_new_tfilter() got some non-obvious bugs
after I introduced replay there without re-setting some of the required
variables.

>  	err = nlmsg_parse_deprecated(n, sizeof(*tcm), tca, TCA_MAX,
>  				     rtm_tca_policy, extack);
>  	if (err < 0)
> @@ -1515,8 +1530,11 @@ static int tc_get_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
>  			return -ENOENT;
>  		}
>  		err = qdisc_graft(dev, p, skb, n, clid, NULL, q, extack);
> -		if (err != 0)
> +		if (err != 0) {
> +			if (err == -EAGAIN)
> +				goto replay;
>  			return err;
> +		}
>  	} else {
>  		qdisc_notify(net, skb, n, clid, NULL, q, NULL);
>  	}
> @@ -1704,6 +1722,8 @@ static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n,
>  	if (err) {
>  		if (q)
>  			qdisc_put(q);
> +		if (err == -EAGAIN)
> +			goto replay;
>  		return err;
>  	}
>  
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 37e41f972f69..e14ed47f961c 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);