[PATCH net 1/2] net: shaper: protect late read accesses to the hierarchy

[Jakub Kicinski <kuba@kernel.org>]

We look up a netdev during prep of Netlink ops (pre- callbacks)
and take a ref to it. Then later in the body of the callback
we take its lock or RCU which are the actual protections.

This is not proper, a conversion from a ref to a locked netdev
must include a liveness check (a check if the netdev hasn't been
unregistered already). Fix the read cases (those under RCU).
Writes needs a separate change to protect from creating the
hierarchy after flush has already run.

Fixes: 4b623f9f0f59 ("net-shapers: implement NL get operation")
Reported-by: Paul Moses <p@1g4.org>
Link: https://lore.kernel.org/20260309173450.538026-1-p@1g4.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 net/shaper/shaper.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/net/shaper/shaper.c b/net/shaper/shaper.c
index 3fd6629cb999..6b4c87e12f1f 100644
--- a/net/shaper/shaper.c
+++ b/net/shaper/shaper.c
@@ -65,6 +65,21 @@ net_shaper_hierarchy(struct net_shaper_binding *binding)
 	return NULL;
 }
 
+static struct net_shaper_hierarchy *
+net_shaper_hierarchy_rcu(struct net_shaper_binding *binding)
+{
+	/* Readers look up the device and take a ref, then take RCU lock
+	 * later at which point netdev may have been unregistered and flushed.
+	 * READ_ONCE() pairs with WRITE_ONCE() in net_shaper_hierarchy_setup.
+	 */
+	if (binding->type == NET_SHAPER_BINDING_TYPE_NETDEV &&
+	    READ_ONCE(binding->netdev->reg_state) <= NETREG_REGISTERED)
+		return READ_ONCE(binding->netdev->net_shaper_hierarchy);
+
+	/* No other type supported yet. */
+	return NULL;
+}
+
 static const struct net_shaper_ops *
 net_shaper_ops(struct net_shaper_binding *binding)
 {
@@ -251,9 +266,10 @@ static struct net_shaper *
 net_shaper_lookup(struct net_shaper_binding *binding,
 		  const struct net_shaper_handle *handle)
 {
-	struct net_shaper_hierarchy *hierarchy = net_shaper_hierarchy(binding);
 	u32 index = net_shaper_handle_to_index(handle);
+	struct net_shaper_hierarchy *hierarchy;
 
+	hierarchy = net_shaper_hierarchy_rcu(binding);
 	if (!hierarchy || xa_get_mark(&hierarchy->shapers, index,
 				      NET_SHAPER_NOT_VALID))
 		return NULL;
@@ -778,17 +794,19 @@ int net_shaper_nl_get_dumpit(struct sk_buff *skb,
 
 	/* Don't error out dumps performed before any set operation. */
 	binding = net_shaper_binding_from_ctx(ctx);
-	hierarchy = net_shaper_hierarchy(binding);
-	if (!hierarchy)
-		return 0;
 
 	rcu_read_lock();
+	hierarchy = net_shaper_hierarchy_rcu(binding);
+	if (!hierarchy)
+		goto out_unlock;
+
 	for (; (shaper = xa_find(&hierarchy->shapers, &ctx->start_index,
 				 U32_MAX, XA_PRESENT)); ctx->start_index++) {
 		ret = net_shaper_fill_one(skb, binding, shaper, info);
 		if (ret)
 			break;
 	}
+out_unlock:
 	rcu_read_unlock();
 
 	return ret;
-- 
2.53.0