[PATCH net v2] net: devmem: use READ_ONCE/WRITE_ONCE on binding->dev

[Bobby Eshleman <bobbyeshleman@gmail.com>]

From: Bobby Eshleman <bobbyeshleman@meta.com>

binding->dev is protected on the write-side in
mp_dmabuf_devmem_uninstall() against concurrent writes, but due to the
concurrent bare reads in net_devmem_get_binding() and
validate_xmit_unreadable_skb() it should be wrapped in a
READ_ONCE/WRITE_ONCE pair to make sure no compiler optimizations play
with the underlying register in unforeseen ways.

Doesn't present a critical bug because the known compiler optimizations
don't result in bad behavior. There is no tearing on u64, and load
omissions/invented loads would only break if additional binding->dev
references were inlined together (they aren't right now).

This just more strictly follows the linux memory model (i.e.,
"Lock-Protected Writes With Lockless Reads" in
tools/memory-model/Documentation/access-marking.txt).

Fixes: bd61848900bf ("net: devmem: Implement TX path")
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
Changes in v2:
- add ASSERT_EXCLUSIVE_WRITER() to mark write-side critical section
- add READ_ONCE() in validate_xmit_unreadable_skb() too
- Link to v1: https://lore.kernel.org/r/20260223-devmem-membar-fix-v1-1-37dcae1e49f8@meta.com
---
 net/core/dev.c    | 2 +-
 net/core/devmem.c | 6 ++++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index c1a9f7fdcffa..9036fcf9f965 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3987,7 +3987,7 @@ static struct sk_buff *validate_xmit_unreadable_skb(struct sk_buff *skb,
 	if (shinfo->nr_frags > 0) {
 		niov = netmem_to_net_iov(skb_frag_netmem(&shinfo->frags[0]));
 		if (net_is_devmem_iov(niov) &&
-		    net_devmem_iov_binding(niov)->dev != dev)
+		    READ_ONCE(net_devmem_iov_binding(niov)->dev) != dev)
 			goto out_free;
 	}
 
diff --git a/net/core/devmem.c b/net/core/devmem.c
index 8c9aad776bf4..69d79aee07ef 100644
--- a/net/core/devmem.c
+++ b/net/core/devmem.c
@@ -396,7 +396,8 @@ struct net_devmem_dmabuf_binding *net_devmem_get_binding(struct sock *sk,
 	 * net_device.
 	 */
 	dst_dev = dst_dev_rcu(dst);
-	if (unlikely(!dst_dev) || unlikely(dst_dev != binding->dev)) {
+	if (unlikely(!dst_dev) ||
+	    unlikely(dst_dev != READ_ONCE(binding->dev))) {
 		err = -ENODEV;
 		goto out_unlock;
 	}
@@ -513,7 +514,8 @@ static void mp_dmabuf_devmem_uninstall(void *mp_priv,
 			xa_erase(&binding->bound_rxqs, xa_idx);
 			if (xa_empty(&binding->bound_rxqs)) {
 				mutex_lock(&binding->lock);
-				binding->dev = NULL;
+				ASSERT_EXCLUSIVE_WRITER(binding->dev);
+				WRITE_ONCE(binding->dev, NULL);
 				mutex_unlock(&binding->lock);
 			}
 			break;

---
base-commit: 9439a661c2e80485406ce2c90b107ca17858382d
change-id: 20260223-devmem-membar-fix-3a5cd9618f8a

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@meta.com>