net: devmem: refresh devmem TX dst in case of route invalidation

[ Netdev upstream commit 6a2108c78069fda000729b88c97b1eba0405e6d7 ]
[ This needs an updated sha from upstream once the commit is merged]

The zero-copy Device Memory (Devmem) transmit path
relies on the socket's route cache (`dst_entry`) to
validate that the packet is being sent via the network
device to which the DMA buffer was bound.

However, this check incorrectly fails and returns `-ENODEV`
if the socket's route cache entry (`dst`) is merely missing
or expired (`dst == NULL`). This scenario is observed during
network events, such as when flow steering rules are deleted,
leading to a temporary route cache invalidation.

This patch fixes -ENODEV error for `net_devmem_get_binding()`
by doing the following:

1.  It attempts to rebuild the route via `rebuild_header()`
if the route is initially missing (`dst == NULL`). This
allows the TCP/IP stack to recover from transient route
cache misses.
2.  It uses `rcu_read_lock()` and `dst_dev_rcu()` to safely
access the network device pointer (`dst_dev`) from the
route, preventing use-after-free conditions if the
device is concurrently removed.
3.  It maintains the critical safety check by validating
that the retrieved destination device (`dst_dev`) is
exactly the device registered in the Devmem binding
(`binding->dev`).

These changes prevent unnecessary ENODEV failures while
maintaining the critical safety requirement that the
Devmem resources are only used on the bound network device.

BUG=b/452457749
TEST=TcpxFunctionalTest
RELEASE_NOTE=Fixed a TCPX bug which would sometimes incorrectly report devices as being missing when route cache entries were missing or invalidated.

Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Reported-by: Vedant Mathur <vedantmathur@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Fixes: bd61848900bf ("net: devmem: Implement TX path")
Change-Id: I14b2801731d7e24b224f50a3ee20379e58a4b350
Link: https://git.kernel.org/netdev/net/c/6a2108c78069
Signed-off-by: Shivaji Kant <shivajikant@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/116522
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>

net/core/devmem.c

diff --git a/net/core/devmem.c b/net/core/devmem.c
index 0b4ca56..812a754 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -17,6 +17,7 @@
 #include <net/page_pool/helpers.h>
 #include <net/page_pool/memory_provider.h>
 #include <net/sock.h>
+#include <net/tcp.h>
 #include <trace/events/page_pool.h>
 
 #include "devmem.h"
@@ -370,7 +371,8 @@ struct net_devmem_dmabuf_binding *net_devmem_get_binding(struct sock *sk,
 							 unsigned int dmabuf_id)
 {
 	struct net_devmem_dmabuf_binding *binding;
-	struct dst_entry *dst = __sk_dst_get(sk);
+	struct net_device *dst_dev;
+	struct dst_entry *dst;
 	int err = 0;
 
 	binding = net_devmem_lookup_dmabuf(dmabuf_id);
@@ -379,16 +381,35 @@ struct net_devmem_dmabuf_binding *net_devmem_get_binding(struct sock *sk,
 		goto out_err;
 	}
 
+	rcu_read_lock();
+	dst = __sk_dst_get(sk);
+	/* If dst is NULL (route expired), attempt to rebuild it. */
+	if (unlikely(!dst)) {
+		if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk)) {
+			err = -EHOSTUNREACH;
+			goto out_unlock;
+		}
+		dst = __sk_dst_get(sk);
+		if (unlikely(!dst)) {
+			err = -ENODEV;
+			goto out_unlock;
+		}
+	}
+
 	/* The dma-addrs in this binding are only reachable to the corresponding
 	 * net_device.
 	 */
-	if (!dst || !dst->dev || dst->dev->ifindex != binding->dev->ifindex) {
+	dst_dev = dst_dev_rcu(dst);
+	if (unlikely(!dst_dev) || unlikely(dst_dev != binding->dev)) {
 		err = -ENODEV;
-		goto out_err;
+		goto out_unlock;
 	}
 
+	rcu_read_unlock();
 	return binding;
 
+out_unlock:
+	rcu_read_unlock();
 out_err:
 	if (binding)
 		net_devmem_dmabuf_binding_put(binding);