xsk_init() previously registered the PF_XDP socket family before the
per-net subsystem and other prerequisites (netdevice notifier, caches)
were fully initialized.

This exposed .create = xsk_create() to user space while per-netns
state (net->xdp.lock/list) was still uninitialized. A task with
CAP_NET_RAW could trigger this during boot/module load by calling
socket(PF_XDP, SOCK_RAW, 0) concurrently with xsk_init(), leading
to a NULL pointer dereference or use-after-free in the list manipulation.

To fix this, move sock_register() to the end of the initialization
sequence, ensuring that all required kernel structures are ready before
exposing the AF_XDP interface to userspace.

Accordingly, reorder the error unwind path to ensure proper cleanup
in reverse order of initialization. Also, explicitly add
kmem_cache_destroy() in the error path to prevent leaking
xsk_tx_generic_cache if the registration fails.

Fixes: c0c77d8fb787 ("xsk: add user memory registration support sockopt")
Signed-off-by: Kery Qi <qikeyu2017@gmail.com>
---
 net/xdp/xsk.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index f093c3453f64..58e9c61c29e0 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -23,6 +23,7 @@
 #include <linux/netdevice.h>
 #include <linux/rculist.h>
 #include <linux/vmalloc.h>
+#include <linux/slab.h>
 #include <net/xdp_sock_drv.h>
 #include <net/busy_poll.h>
 #include <net/netdev_lock.h>
@@ -1922,10 +1923,6 @@ static int __init xsk_init(void)
 	if (err)
 		goto out;
 
-	err = sock_register(&xsk_family_ops);
-	if (err)
-		goto out_proto;
-
 	err = register_pernet_subsys(&xsk_net_ops);
 	if (err)
 		goto out_sk;
@@ -1942,16 +1939,21 @@ static int __init xsk_init(void)
 		goto out_unreg_notif;
 	}
 
+	err = sock_register(&xsk_family_ops);
+	if (err)
+		goto out_proto;
+
 	return 0;
 
+out_proto:
+	proto_unregister(&xsk_proto);
+	kmem_cache_destroy(xsk_tx_generic_cache);
 out_unreg_notif:
 	unregister_netdevice_notifier(&xsk_netdev_notifier);
 out_pernet:
 	unregister_pernet_subsys(&xsk_net_ops);
 out_sk:
 	sock_unregister(PF_XDP);
-out_proto:
-	proto_unregister(&xsk_proto);
 out:
 	return err;
 }
-- 
2.34.1

> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index f093c3453f64..58e9c61c29e0 100644

[ ... ]

> @@ -1922,10 +1923,6 @@ static int __init xsk_init(void)
>  	if (err)
>  		goto out;
>
> -	err = sock_register(&xsk_family_ops);
> -	if (err)
> -		goto out_proto;
> -
>  	err = register_pernet_subsys(&xsk_net_ops);
>  	if (err)
>  		goto out_sk;

[ ... ]

> @@ -1942,16 +1939,21 @@ static int __init xsk_init(void)
>  		goto out_unreg_notif;
>  	}
>
> +	err = sock_register(&xsk_family_ops);
> +	if (err)
> +		goto out_proto;
> +
>  	return 0;
>
> +out_proto:
> +	proto_unregister(&xsk_proto);
> +	kmem_cache_destroy(xsk_tx_generic_cache);
>  out_unreg_notif:
>  	unregister_netdevice_notifier(&xsk_netdev_notifier);
>  out_pernet:
>  	unregister_pernet_subsys(&xsk_net_ops);
>  out_sk:
>  	sock_unregister(PF_XDP);
                ^^^^^^^^^^^^^^^^

With sock_register() now at the end of xsk_init(), the out_sk label is
reached via goto from register_pernet_subsys(), register_netdevice_notifier(),
or kmem_cache_create() failures - all of which occur before sock_register()
is called.

Shouldn't out_sk be removed entirely, and the gotos redirected? For example,
if register_pernet_subsys() fails, we goto out_sk and call sock_unregister()
on a family that was never registered. The same applies to the other error
paths that fall through to out_sk.

While sock_unregister() on an unregistered family appears to be benign (it
just sets NULL to NULL), it will print "NET: Unregistered PF_XDP protocol
family" to the kernel log even though it was never registered in the first
place.

> -out_proto:
> -	proto_unregister(&xsk_proto);
>  out:
>  	return err;
>  }


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20848468294