[PATCH net v2 2/4] net: devmem: detach TX bindings on NETDEV_UNREGISTER

[Mina Almasry <almasrymina@google.com>]

TX devmem bindings (NETDEV_CMD_BIND_TX) have no bound RX queues, so
mp_dmabuf_devmem_uninstall() never runs when binding->dev or
binding->vdev unregisters. As a result, binding->dev and binding->vdev
dangle pointing to freed struct net_device memory, and the dma_buf
remains attached and mapped after device removal.

Closing the netlink socket after netdev unregistration dereferences the
freed struct net_device in netdev_nl_sock_priv_destroy():

  BUG: unable to handle page fault for address: ffff88824ca9b000
  Oops: Oops: 0000 [#1] SMP KASAN NOPTI
  RIP: 0010:netdev_nl_sock_priv_destroy+0xbd/0x1e0
  Call Trace:
   genl_release+0xf5/0x190
   netlink_release+0xd01/0x18d0
   __sock_release+0xb0/0x270
   sock_close+0x18/0x20

Extend net_devmem_dev_unregister() to TX bindings, including ones
whose netlink socket is already closed but which TCP retransmits keep
alive:
- When binding->dev (the physical DMA-capable device) unregisters,
  clear the binding's NET_DEVMEM_BINDING_LIVE mark, clear both
  binding->dev and binding->vdev, wait for in-flight RCU/TX readers via
  synchronize_net(), and unmap the dma_buf under binding->lock.
- When only binding->vdev (virtual device) unregisters while
  binding->dev lives, only clear binding->vdev so no new TX sends match
  the virtual device while allowing in-flight TX packets on binding->dev
  to finish DMA safely.

Fixes: bd61848900bf ("net: devmem: Implement TX path")
Fixes: 1abe839b34ae ("net: devmem: support TX over NETMEM_TX_NO_DMA devices")
Cc: Kaiyuan Zhang <kaiyuanz@google.com>
Cc: Stanislav Fomichev <sdf@fomichev.me>
Cc: Bobby Eshleman <bobbyeshleman@meta.com>
Cc: Tariq Toukan <tariqt@nvidia.com>
Cc: Kaifeng Wang <kaifengw@google.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
---
v2:
- Build on the net_devmem_dev_unregister() walk from the previous patch,
  so unbound TX bindings are also detached (Sashiko). Clear the
  binding's LIVE mark instead of erasing its ID.
- The walk holds net_devmem_lock, which the final free also takes
  before erasing the binding, instead of using RCU and
  percpu_ref_tryget(), so it no longer takes a binding reference
  (Stanislav).
---
 net/core/devmem.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/net/core/devmem.c b/net/core/devmem.c
index 02d607cc87217..a1c029f6ae47f 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -192,6 +192,36 @@ net_devmem_dev_unregister_rx(struct net_devmem_dmabuf_binding *binding,
 	mutex_unlock(&binding->lock);
 }
 
+static void
+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) {
+		xa_clear_mark(&net_devmem_dmabuf_bindings, xa_idx,
+			      NET_DEVMEM_BINDING_LIVE);
+
+		mutex_lock(&binding->lock);
+		WRITE_ONCE(binding->dev, NULL);
+		WRITE_ONCE(binding->vdev, NULL);
+		mutex_unlock(&binding->lock);
+
+		/* Ensure no tx net_devmem_lookup_dmabuf() or
+		 * validate_xmit_unreadable_skb() are in flight after detach.
+		 */
+		synchronize_net();
+
+		mutex_lock(&binding->lock);
+		net_devmem_dmabuf_binding_unmap(binding);
+		mutex_unlock(&binding->lock);
+	} else if (READ_ONCE(binding->vdev) == dev) {
+		mutex_lock(&binding->lock);
+		WRITE_ONCE(binding->vdev, NULL);
+		mutex_unlock(&binding->lock);
+
+		synchronize_net();
+	}
+}
+
 void net_devmem_dev_unregister(struct net_device *dev)
 {
 	struct net_devmem_dmabuf_binding *binding;
@@ -201,6 +231,8 @@ void net_devmem_dev_unregister(struct net_device *dev)
 	xa_for_each(&net_devmem_dmabuf_bindings, xa_idx, binding) {
 		if (binding->direction == DMA_FROM_DEVICE)
 			net_devmem_dev_unregister_rx(binding, xa_idx, dev);
+		else
+			net_devmem_dev_unregister_tx(binding, xa_idx, dev);
 	}
 	mutex_unlock(&net_devmem_lock);
 }
-- 
2.56.0.385.gd3acb90ef8-goog