[PATCH bpf-next v4] xsk: Fix circular locking dependency between &net->xdp.lock, &xs->mutex, and netdev_lock_ops
From: Khawar Ahemad
Date: Wed Aug 26 2026 - 13:48:39 EST
syzbot reported a circular locking dependency involving &net->xdp.lock,
&port->pnodes_lock, netdev_lock_ops(), and &xs->mutex:
-> #3 (&net->xdp.lock):
xsk_notifier+0x3d/0x2c0 net/xdp/xsk.c:2106
ipvlan_device_event+0x310/0x4e0 drivers/net/ipvlan/ipvlan_main.c:834
unregister_netdevice_many_notify+0x808/0x18b0 net/core/dev.c:12518
-> #2 (&port->pnodes_lock):
ipvlan_device_event+0x85/0x4e0 drivers/net/ipvlan/ipvlan_main.c:795
notifier_call_chain+0xb5/0x410 kernel/notifier.c:85
-> #1 (&dev_instance_lock_key / netdev_lock_ops):
netdev_lock_ops include/net/netdev_lock.h:42 [inline]
xsk_bind+0x331/0x11d0 net/xdp/xsk.c:1627
-> #0 (&xs->mutex):
xsk_diag_fill net/xdp/xsk_diag.c:113 [inline]
xsk_diag_dump+0x2e0/0x4e0 net/xdp/xsk_diag.c:166
The cycle exists through the following dependency chain:
1. xsk_diag_dump() acquired &xs->mutex while holding &net->xdp.lock (#0).
2. xsk_bind() acquired netdev_lock_ops() while holding &xs->mutex (#1).
3. Device unregistration in ipvlan_device_event() acquired
&port->pnodes_lock (#2) and called xsk_notifier(), which acquired
&net->xdp.lock (#3).
Break the circular dependency by decoupling the locking in xsk_diag_dump()
and xsk_notifier():
1. In xsk_diag_dump(), avoid holding &net->xdp.lock while calling
xsk_diag_fill(). Instead, locate the target socket under &net->xdp.lock,
take a temporary socket reference via sock_hold(), release
&net->xdp.lock, and call xsk_diag_fill() (which acquires &xs->mutex)
with sock_put().
To preserve dump continuation across buffer exhaustion, distinguish
-ENOENT (when an unbound socket is skipped) from -EMSGSIZE (when the
skb is full and the cursor must be retained for the next dump callback).
2. In xsk_notifier(), split device unregistration into two phases:
- First, unbind all matching sockets under &net->xdp.lock and
&xs->mutex.
- Then, release &net->xdp.lock and perform device queue teardown by
sweeping the device queues via xsk_get_pool_from_qid() and calling
xp_clear_dev(pool) outside all AF_XDP locks.
Fixes: 975b11ae9077 ("xsk: add socket allocate, create and bind")
Reported-by: syzbot+aa48b5fe7bfda62d1682@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=aa48b5fe7bfda62d1682
Signed-off-by: Khawar Ahemad <ahemadkhawar123@xxxxxxxxx>
---
v3 -> v4:
- Rebase cleanly on latest bpf-next master to resolve merge conflict.
- Update commit message to accurately describe the full 4-lock dependency
chain (&net->xdp.lock, &port->pnodes_lock, netdev_lock_ops, &xs->mutex)
from the syzbot report.
- Link to v3: https://lore.kernel.org/bpf/20260826173019.2917-1-ahemadkhawar123@xxxxxxxxx/
v2 -> v3:
- Fix direct AB-BA lock inversion in xsk_notifier() by performing device
queue sweeps via xsk_get_pool_from_qid() outside &net->xdp.lock.
- Eliminate &net->xdp.lock -> &xs->mutex in xsk_diag_dump() by taking a
temporary socket reference under &net->xdp.lock and releasing the lock
prior to xsk_diag_fill().
- Distinguish -ENOENT (skipped unbound socket) from -EMSGSIZE (buffer
exhaustion) to preserve dump continuation without infinite loops.
- Link to v2: https://lore.kernel.org/bpf/20260826162110.99879-1-ahemadkhawar123@xxxxxxxxx/
v1 -> v2:
- Avoid reordering locks in xsk_bind() to preserve errno precedence.
- Link to v1: https://lore.kernel.org/bpf/20260825152152.86092-1-ahemadkhawar123@xxxxxxxxx/
net/xdp/xsk.c | 16 +++++++++---
net/xdp/xsk_diag.c | 65 +++++++++++++++++++++++++++++++---------------
2 files changed, 57 insertions(+), 24 deletions(-)
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 7855ee09c4..e72344fccb 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -2099,7 +2099,9 @@ static int xsk_notifier(struct notifier_block *this,
{
struct net_device *dev = netdev_notifier_info_to_dev(ptr);
struct net *net = dev_net(dev);
+ unsigned int max_queues;
struct sock *sk;
+ u16 qid;
switch (msg) {
case NETDEV_UNREGISTER:
@@ -2114,13 +2116,21 @@ static int xsk_notifier(struct notifier_block *this,
sk_error_report(sk);
xsk_unbind_dev(xs);
-
- /* Clear device references. */
- xp_clear_dev(xs->pool);
}
mutex_unlock(&xs->mutex);
}
mutex_unlock(&net->xdp.lock);
+
+ /* Clear device references outside AF_XDP locks to avoid
+ * lock inversion with netdev_lock_ops().
+ */
+ max_queues = max(dev->real_num_rx_queues, dev->real_num_tx_queues);
+ for (qid = 0; qid < max_queues; qid++) {
+ struct xsk_buff_pool *pool = xsk_get_pool_from_qid(dev, qid);
+
+ if (pool)
+ xp_clear_dev(pool);
+ }
break;
}
return NOTIFY_DONE;
diff --git a/net/xdp/xsk_diag.c b/net/xdp/xsk_diag.c
index 0170363eb5..bad0b13064 100644
--- a/net/xdp/xsk_diag.c
+++ b/net/xdp/xsk_diag.c
@@ -97,6 +97,7 @@ static int xsk_diag_fill(struct sock *sk, struct sk_buff *nlskb,
struct xdp_sock *xs = xdp_sk(sk);
struct xdp_diag_msg *msg;
struct nlmsghdr *nlh;
+ int err = -EMSGSIZE;
nlh = nlmsg_put(nlskb, portid, seq, SOCK_DIAG_BY_FAMILY, sizeof(*msg),
flags);
@@ -111,8 +112,10 @@ static int xsk_diag_fill(struct sock *sk, struct sk_buff *nlskb,
sock_diag_save_cookie(sk, msg->xdiag_cookie);
mutex_lock(&xs->mutex);
- if (READ_ONCE(xs->state) == XSK_UNBOUND)
+ if (READ_ONCE(xs->state) == XSK_UNBOUND) {
+ err = -ENOENT;
goto out_nlmsg_trim;
+ }
if ((req->xdiag_show & XDP_SHOW_INFO) && xsk_diag_put_info(xs, nlskb))
goto out_nlmsg_trim;
@@ -145,7 +148,7 @@ static int xsk_diag_fill(struct sock *sk, struct sk_buff *nlskb,
out_nlmsg_trim:
mutex_unlock(&xs->mutex);
nlmsg_cancel(nlskb, nlh);
- return -EMSGSIZE;
+ return err;
}
static int xsk_diag_dump(struct sk_buff *nlskb, struct netlink_callback *cb)
@@ -153,28 +156,48 @@ static int xsk_diag_dump(struct sk_buff *nlskb, struct netlink_callback *cb)
struct xdp_diag_req *req = nlmsg_data(cb->nlh);
struct net *net = sock_net(nlskb->sk);
int num = 0, s_num = cb->args[0];
- struct sock *sk;
-
- mutex_lock(&net->xdp.lock);
-
- sk_for_each(sk, &net->xdp.list) {
- if (!net_eq(sock_net(sk), net))
- continue;
- if (num++ < s_num)
- continue;
-
- if (xsk_diag_fill(sk, nlskb, req,
- sk_user_ns(NETLINK_CB(cb->skb).sk),
- NETLINK_CB(cb->skb).portid,
- cb->nlh->nlmsg_seq, NLM_F_MULTI,
- sock_i_ino(sk)) < 0) {
- num--;
- break;
+ struct sock *sk, *target_sk;
+ int err;
+
+ for (;;) {
+ target_sk = NULL;
+ num = 0;
+
+ mutex_lock(&net->xdp.lock);
+ sk_for_each(sk, &net->xdp.list) {
+ if (!net_eq(sock_net(sk), net))
+ continue;
+ if (num++ == s_num) {
+ sock_hold(sk);
+ target_sk = sk;
+ break;
+ }
}
+ mutex_unlock(&net->xdp.lock);
+
+ if (!target_sk)
+ break;
+
+ err = xsk_diag_fill(target_sk, nlskb, req,
+ sk_user_ns(NETLINK_CB(cb->skb).sk),
+ NETLINK_CB(cb->skb).portid,
+ cb->nlh->nlmsg_seq, NLM_F_MULTI,
+ sock_i_ino(target_sk));
+ sock_put(target_sk);
+
+ /*
+ * xsk_diag_fill() returns:
+ * 0: entry added successfully.
+ * -ENOENT: socket is unbound, skip it.
+ * -EMSGSIZE: skb is full, retry this socket on the next dump callback.
+ */
+ if (err == -EMSGSIZE)
+ break;
+
+ s_num++;
}
- mutex_unlock(&net->xdp.lock);
- cb->args[0] = num;
+ cb->args[0] = s_num;
return nlskb->len;
}
--
2.54.0 (Apple Git-157)