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issue(s) to consider.
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- [High] The TX-binding dangling `binding->dev` use-after-free can still
happen through a race in the leased-queue (netkit) bind-tx path.
--
net: devmem: detach TX bindings on NETDEV_UNREGISTER
This extends net_devmem_dev_unregister() to cover TX devmem bindings.
When binding->dev unregisters, the patch clears the binding's LIVE mark
and both dev and vdev, then unmaps the dma_buf after synchronize_net().
When only binding->vdev unregisters, it clears just binding->vdev.
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index 93cc2815d935a..dd07203076808 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -192,9 +192,55 @@ net_devmem_dev_unregister_rx(struct
> net_devmem_dmabuf_binding *binding,
> mutex_unlock(&binding->lock);
> }
>
> +static bool
> +net_devmem_dev_unregister_tx(struct net_devmem_dmabuf_binding *binding,
> + unsigned long xa_idx, struct net_device *dev)
> +{
> + if (READ_ONCE(binding->dev) == dev) {
[ ... ]
> + return true;
> + }
> +
> + if (READ_ONCE(binding->vdev) == dev) {
> + mutex_lock(&binding->lock);
> + WRITE_ONCE(binding->vdev, NULL);
> + mutex_unlock(&binding->lock);
> + }
[Severity: High]
Can a TX binding still end up with a dangling binding->dev through the
leased queue (netkit) bind-tx path?
In netdev_nl_bind_tx_doit(), when the target is a NETMEM_TX_NO_DMA
leasee, netdev_find_netmem_tx_dev() finds the physical device. It reads
the lease without taking the physical device's lock. It also only checks
netif_device_present(), and the unregister path never clears that:
net/core/netdev-genl.c:netdev_find_netmem_tx_dev() {
...
phys_dev = lease_rxq->dev;
if (netif_device_present(phys_dev) &&
phys_dev->netmem_tx == NETMEM_TX_DMA)
return phys_dev;
...
}
The bind then goes ahead without rechecking bind_dev->reg_state:
net/core/netdev-genl.c:netdev_nl_bind_tx_doit() {
...
if (bind_dev != netdev)
netdev_lock(bind_dev);
dma_dev = netdev_queue_get_dma_dev(bind_dev, 0, NETDEV_QUEUE_TYPE_TX);
binding = net_devmem_bind_dmabuf(bind_dev,
...
}
On the direct path, netdev_get_by_index_lock()->netdev_put_lock() rejects
reg_state > NETREG_REGISTERED. Nothing does the same check for bind_dev
here.
The sequence seems to be:
1. The physical device P unregisters.
unregister_netdevice_many_notify() sets P->reg_state to
NETREG_UNREGISTERING and calls the NETDEV_UNREGISTER notifiers.
2. The netdev-genl notifier runs before netkit's, because it was
registered earlier with equal priority. It calls
net_devmem_dev_unregister(P) under netdev_lock(P).
3. A concurrent unprivileged bind-tx on netkit already holds netkit's
lock and is blocked in netdev_lock(P). It now continues. The lease is
still in place, so it creates a binding with dev = P and
vdev = netkit, maps the dmabuf, and marks the binding LIVE.
4. netkit_check_lease_unregister(P) unregisters netkit. In
net_devmem_dev_unregister(netkit)->net_devmem_dev_unregister_tx(),
binding->dev is P, so only the branch above runs and only
binding->vdev is cleared.
5. netdev_run_todo() sets P->reg_state to NETREG_UNREGISTERED. Any
rebroadcast NETDEV_UNREGISTER then returns early at:
if (dev->reg_state != NETREG_UNREGISTERING)
return;
net_devmem_bind_dmabuf() sets binding->dev = dev without taking a
netdev_hold(), so P can be freed.
6. When the socket closes, netdev_nl_sock_priv_destroy() reads the stale
binding->dev and calls netdev_hold() and netdev_lock() on it. This is
the same oops quoted in the commit message. The dma_buf also stays
attached to the removed device until the final free.
NETDEV_CMD_BIND_TX is unprivileged. Hitting this needs a race with the
physical device's unregistration. That is still the path the commit
message and the 1abe839b34ae Fixes: tag say this patch fixes. Later
patches in the series don't appear to touch net/ or drivers/net/netkit.c
to address it.
Would it make sense to recheck bind_dev->reg_state after
netdev_lock(bind_dev) in netdev_nl_bind_tx_doit()? Extending the
unregister walk doesn't look like it can close this window on its own.
[ ... ]
--
Sashiko AI review ·
https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261014161754.3782346-1-almasrymina%40google.com
