[PATCH net v4] net: pin protocol module before inet socket allocation

[Chengfeng Ye <nicoyip.dev@gmail.com>]

inet_create() and inet6_create() look up the protocol under RCU, then drop
RCU before using the resulting proto. A concurrent module unload can
unregister the protosw, wait for the RCU readers, and destroy the proto's
slab before sk_alloc() uses it. sk_alloc() may sleep, so the RCU read-side
critical section cannot simply be extended across the allocation.

The reported failure occurred during concurrent L2TP socket creation and
module unload:

  Oops: general protection fault, probably for non-canonical address
        0xffff1108974182a0
  KASAN: maybe wild-memory-access in range
         [0xfff8a844ba0c1500-0xfff8a844ba0c1507]
  RIP: kmem_cache_alloc_noprof+0x63/0x370
  Call Trace:
   sk_prot_alloc+0x74/0x2c0
   sk_alloc+0x2b/0x6c0
   inet_create+0x2cd/0xd40
   __sock_create+0x1c3/0x430
   __sys_socket+0x116/0x1d0

Cache the proto and its owner under RCU and acquire a temporary module
reference before dropping RCU. This prevents normal module unload while
the proto is in use. sk_prot_alloc() takes the socket's own reference;
drop the temporary reference on every exit after allocation. Use the
cached proto for the IPv6 backlog callback as well.

Use try_module_get() with its existing initialization semantics. SCTP
creates its kernel control socket during module initialization, so
rejecting MODULE_STATE_COMING would prevent modular SCTP from loading.
Failed module initialization can still unwind published protocols despite
module references; that separate publication/unwind lifetime issue is not
fixed here.

Fixes: a79af59efd20 ("[NET]: Fix module reference counts for loadable protocol modules")
Cc: stable@vger.kernel.org
Assisted-by: GPT-6-Astra
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
Changes in v4:
- Remove the MODULE_STATE_COMING rejection. SCTP creates its kernel control
  socket from module init and must be allowed to use the published protosw.
- Keep the owner cached under RCU and use the cached proto for IPv6 backlog
  setup, retaining those v3 changes.
- Scope the module-reference guarantee to normal module unload. The earlier
  review's initialization-failure unwind concern remains a separate issue;
  rejecting all initializing modules is not a compatible solution.
- Rebase onto net/main at a7bfaba4823e.

v3: https://lore.kernel.org/r/20260904111514.584264-1-nicoyip.dev@gmail.com/
Review: https://lore.kernel.org/r/178887699602.219967.13594894510710999619@kernel.org/
v2: https://lore.kernel.org/r/20260825172349.232794-1-nicoyip.dev@gmail.com/
v1: https://lore.kernel.org/r/20260823171311.3857087-1-nicoyip.dev@gmail.com/

 net/ipv4/af_inet.c  | 14 +++++++++++---
 net/ipv6/af_inet6.c | 16 ++++++++++++----
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 32d006c1a8ee..c956afa65ec4 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -263,6 +263,7 @@ static int inet_create(struct net *net, struct socket *sock, int protocol,
 	struct inet_protosw *answer;
 	struct inet_sock *inet;
 	struct proto *answer_prot;
+	struct module *answer_owner;
 	unsigned char answer_flags;
 	int try_loading_module = 0;
 	int err;
@@ -322,9 +323,14 @@ static int inet_create(struct net *net, struct socket *sock, int protocol,
 	    !ns_capable(net->user_ns, CAP_NET_RAW))
 		goto out_rcu_unlock;
 
-	sock->ops = answer->ops;
 	answer_prot = answer->prot;
+	answer_owner = answer_prot->owner;
 	answer_flags = answer->flags;
+	if (!try_module_get(answer_owner)) {
+		err = -EPROTONOSUPPORT;
+		goto out_rcu_unlock;
+	}
+	sock->ops = answer->ops;
 	rcu_read_unlock();
 
 	WARN_ON(!answer_prot->slab);
@@ -332,7 +338,7 @@ static int inet_create(struct net *net, struct socket *sock, int protocol,
 	err = -ENOMEM;
 	sk = sk_alloc(net, PF_INET, GFP_KERNEL, answer_prot, kern);
 	if (!sk)
-		goto out;
+		goto out_module_put;
 
 	err = 0;
 	if (INET_PROTOSW_REUSE & answer_flags)
@@ -398,6 +404,8 @@ static int inet_create(struct net *net, struct socket *sock, int protocol,
 		if (err)
 			goto out_sk_release;
 	}
+out_module_put:
+	module_put(answer_owner);
 out:
 	return err;
 out_rcu_unlock:
@@ -406,7 +414,7 @@ static int inet_create(struct net *net, struct socket *sock, int protocol,
 out_sk_release:
 	sk_common_release(sk);
 	sock->sk = NULL;
-	goto out;
+	goto out_module_put;
 }
 
 
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index 282912a11999..9db60af6f38d 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -110,6 +110,7 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 	struct sock *sk;
 	struct inet_protosw *answer;
 	struct proto *answer_prot;
+	struct module *answer_owner;
 	unsigned char answer_flags;
 	int try_loading_module = 0;
 	int err;
@@ -167,9 +168,14 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 	    !ns_capable(net->user_ns, CAP_NET_RAW))
 		goto out_rcu_unlock;
 
-	sock->ops = answer->ops;
 	answer_prot = answer->prot;
+	answer_owner = answer_prot->owner;
 	answer_flags = answer->flags;
+	if (!try_module_get(answer_owner)) {
+		err = -EPROTONOSUPPORT;
+		goto out_rcu_unlock;
+	}
+	sock->ops = answer->ops;
 	rcu_read_unlock();
 
 	WARN_ON(!answer_prot->slab);
@@ -177,7 +183,7 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 	err = -ENOBUFS;
 	sk = sk_alloc(net, PF_INET6, GFP_KERNEL, answer_prot, kern);
 	if (!sk)
-		goto out;
+		goto out_module_put;
 
 	sock_init_data(sock, sk);
 
@@ -201,7 +207,7 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 	sk->sk_family		= PF_INET6;
 	sk->sk_protocol		= protocol;
 
-	sk->sk_backlog_rcv	= answer->prot->backlog_rcv;
+	sk->sk_backlog_rcv	= answer_prot->backlog_rcv;
 
 	inet_sk(sk)->pinet6 = np = inet6_sk_generic(sk);
 	np->hop_limit	= -1;
@@ -251,6 +257,8 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 		if (err)
 			goto out_sk_release;
 	}
+out_module_put:
+	module_put(answer_owner);
 out:
 	return err;
 out_rcu_unlock:
@@ -259,7 +267,7 @@ static int inet6_create(struct net *net, struct socket *sock, int protocol,
 out_sk_release:
 	sk_common_release(sk);
 	sock->sk = NULL;
-	goto out;
+	goto out_module_put;
 }
 
 int __inet6_bind(struct sock *sk, struct sockaddr_unsized *uaddr, int addr_len,

base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.43.0