* [PATCH net v4 1/3] netconsole: finish enabling the target before releasing RTNL
2026-10-21 14:19 [PATCH net v4 0/3] netconsole: fix target enable race and configfs read locking Gustavo Luiz Duarte
@ 2026-10-21 14:19 ` Gustavo Luiz Duarte
2026-10-21 14:19 ` [PATCH net v4 2/3] netconsole: read np.dev under target_list_lock in local_mac_show() Gustavo Luiz Duarte
2026-10-21 14:19 ` [PATCH net v4 3/3] netconsole: recheck device registration after waiting for carrier Gustavo Luiz Duarte
2 siblings, 0 replies; 4+ messages in thread
From: Gustavo Luiz Duarte @ 2026-10-21 14:19 UTC (permalink / raw)
To: Breno Leitao, Andrew Lunn, David S. Miller, Eric Dumazet,
Jakub Kicinski, Paolo Abeni, Stephen Hemminger
Cc: Gustavo Luiz Duarte, netdev, linux-kernel
netcons_netpoll_setup() publishes nt->np.dev inside __netpoll_setup(),
then releases the RTNL and waits for an RCU grace period before
returning. Only after that does its caller store STATE_ENABLED.
A NETDEV_UNREGISTER event landing in that window tears the target down
and we end up storing STATE_ENABLED with a NULL np->dev, leading to a
null-ptr-deref in netconsole_write():
BUG: KASAN: null-ptr-deref in netconsole_write+0xb9/0x7f0
Read of size 8 at addr 00000000000000a8 by task pr/netcon_ext0/135
netconsole_write+0xb9/0x7f0
nbcon_emit_next_record+0x501/0x550
nbcon_emit_one+0x10e/0x170
nbcon_kthread_func+0x2ff/0x3b0
kthread+0x199/0x1e0
Store STATE_ENABLED before dropping the RTNL lock to avoid racing with
netconsole_netdev_event().
Fixes: 2382b15bcc39 ("netconsole: take care of NETDEV_UNREGISTER event")
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
drivers/net/netconsole.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index b358e5c36735..1fb846ad07d1 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -552,13 +552,16 @@ static int netcons_netpoll_setup(struct netconsole_target *nt)
err = __netpoll_setup(np, ndev);
if (err)
goto put;
- rtnl_unlock();
/* Make sure all NAPI polls which started before dev->npinfo
* was visible have exited before we start calling NAPI poll.
* NAPI skips locking if dev->npinfo is NULL.
+ * Hold RTNL until enable is finished so we don't race with
+ * netconsole_netdev_event()
*/
- synchronize_rcu();
+ synchronize_net();
+ nt->state = STATE_ENABLED;
+ rtnl_unlock();
return 0;
@@ -589,7 +592,6 @@ static void resume_target(struct netconsole_target *nt)
return;
}
- nt->state = STATE_ENABLED;
pr_info("network logging resumed on interface %s\n", nt->np.dev_name);
}
@@ -1067,7 +1069,6 @@ static ssize_t enabled_store(struct config_item *item,
goto out_unlock;
}
- nt->state = STATE_ENABLED;
pr_info("network logging started\n");
} else { /* false */
/* We need to disable the netconsole before cleaning it up
@@ -2669,8 +2670,6 @@ static struct netconsole_target *alloc_param_target(char *target_config,
* otherwise, keep the target in the list, but disabled.
*/
goto fail;
- } else {
- nt->state = STATE_ENABLED;
}
populate_configfs_item(nt, cmdline_count);
--
2.56.0
^ permalink raw reply [flat|nested] 4+ messages in thread* [PATCH net v4 2/3] netconsole: read np.dev under target_list_lock in local_mac_show()
2026-10-21 14:19 [PATCH net v4 0/3] netconsole: fix target enable race and configfs read locking Gustavo Luiz Duarte
2026-10-21 14:19 ` [PATCH net v4 1/3] netconsole: finish enabling the target before releasing RTNL Gustavo Luiz Duarte
@ 2026-10-21 14:19 ` Gustavo Luiz Duarte
2026-10-21 14:19 ` [PATCH net v4 3/3] netconsole: recheck device registration after waiting for carrier Gustavo Luiz Duarte
2 siblings, 0 replies; 4+ messages in thread
From: Gustavo Luiz Duarte @ 2026-10-21 14:19 UTC (permalink / raw)
To: Breno Leitao, Andrew Lunn, David S. Miller, Eric Dumazet,
Jakub Kicinski, Paolo Abeni, Stephen Hemminger
Cc: Gustavo Luiz Duarte, netdev, linux-kernel
local_mac_show() loads nt->np.dev and dereferences it with no lock. It
can race with the interface going away and netconsole_netdev_event()
eventually calling netdev_put() and freeing the device before we
dereference it, leading to a UAF.
The window is only a few instructions wide, so I could only reproduce it
with an artificially widened window between the load and the
dereference:
BUG: KASAN: slab-use-after-free in local_mac_show+0x4e/0x80
Read of size 8 at addr ffff88800e990458 by task uaf/130
local_mac_show+0x4e/0x80
configfs_read_iter+0x187/0x260
vfs_read+0x453/0x5a0
Allocated by task 116:
alloc_netdev_mqs+0x78/0x830
rtnl_create_link+0x53b/0x5c0
Freed by task 121:
kfree+0x159/0x420
device_release+0x77/0x120
kobject_put+0xb5/0x160
netdev_run_todo+0x420/0x850
rtnl_dellink+0x259/0x5b0
Two places can free np.dev: (1) netconsole_netdev_event() moves a target
out of STATE_ENABLED under target_list_lock before netdev_put(), and (2)
process_resume_target() releases np.dev under dynamic_netconsole_mutex.
Fixes: 0953864160bd ("[NETPOLL]: no need to store local_mac")
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
drivers/net/netconsole.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 1fb846ad07d1..be7eb632ee42 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -888,10 +888,23 @@ static ssize_t remote_ip_show(struct config_item *item, char *buf)
static ssize_t local_mac_show(struct config_item *item, char *buf)
{
- struct net_device *dev = to_target(item)->np.dev;
- static const u8 bcast[ETH_ALEN] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
+ struct netconsole_target *nt = to_target(item);
+ unsigned long flags;
+ u8 mac[ETH_ALEN];
+
+ eth_broadcast_addr(mac);
+
+ dynamic_netconsole_mutex_lock();
+ /* Hold target_list_lock here so netconsole_netdev_event() doesn't tear
+ * down np.dev from under us.
+ */
+ spin_lock_irqsave(&target_list_lock, flags);
+ if (nt->state == STATE_ENABLED && nt->np.dev)
+ memcpy(mac, nt->np.dev->dev_addr, ETH_ALEN);
+ spin_unlock_irqrestore(&target_list_lock, flags);
+ dynamic_netconsole_mutex_unlock();
- return sysfs_emit(buf, "%pM\n", dev ? dev->dev_addr : bcast);
+ return sysfs_emit(buf, "%pM\n", mac);
}
static ssize_t remote_mac_show(struct config_item *item, char *buf)
--
2.56.0
^ permalink raw reply [flat|nested] 4+ messages in thread* [PATCH net v4 3/3] netconsole: recheck device registration after waiting for carrier
2026-10-21 14:19 [PATCH net v4 0/3] netconsole: fix target enable race and configfs read locking Gustavo Luiz Duarte
2026-10-21 14:19 ` [PATCH net v4 1/3] netconsole: finish enabling the target before releasing RTNL Gustavo Luiz Duarte
2026-10-21 14:19 ` [PATCH net v4 2/3] netconsole: read np.dev under target_list_lock in local_mac_show() Gustavo Luiz Duarte
@ 2026-10-21 14:19 ` Gustavo Luiz Duarte
2 siblings, 0 replies; 4+ messages in thread
From: Gustavo Luiz Duarte @ 2026-10-21 14:19 UTC (permalink / raw)
To: Breno Leitao, Andrew Lunn, David S. Miller, Eric Dumazet,
Jakub Kicinski, Paolo Abeni, Stephen Hemminger
Cc: Gustavo Luiz Duarte, netdev, linux-kernel
netcons_netpoll_setup() drops RTNL while it waits for carrier when the
device is not yet up. If the device is unregistered in that window, the
NETDEV_UNREGISTER notifier runs before np->dev is set and has nothing
to match.
We might then end up setting up netpoll on a device that is being
unregistered while holding a reference to it, stalling
unregister_netdevice() in netdev_wait_allrefs_any().
Add a check to confirm the device is still registered once we retake
RTNL. Release the device reference and return -ENODEV otherwise.
Closes: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261012-netcons-fixes-v3-1-1aac85ed7665@gmail.com
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
---
drivers/net/netconsole.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index be7eb632ee42..bd36ebd334aa 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -534,6 +534,16 @@ static int netcons_netpoll_setup(struct netconsole_target *nt)
rtnl_unlock();
netcons_wait_carrier(np, ndev);
rtnl_lock();
+
+ /* Check that ndev hasn't been unregistered when we released
+ * the RTNL above.
+ */
+ if (ndev->reg_state != NETREG_REGISTERED) {
+ np_err(np, "%s was unregistered while waiting for carrier, aborting\n",
+ netcons_egress_dev(np, buf, sizeof(buf)));
+ err = -ENODEV;
+ goto put;
+ }
}
if (netcons_local_ip_unset(nt)) {
--
2.56.0
^ permalink raw reply [flat|nested] 4+ messages in thread