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

From: Peilin Ye
Date: Mon May 22 2023 - 20:41:58 EST

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: 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>
---
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:
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);
--
2.20.1