[PATCH 1/1] nfc: llcp: Fix race condition in accept_queue lifecycle

[Lee Jones <lee@kernel.org>]

nfc_llcp_socket_release() walks the local sockets list and the listener
socket's accept_queue under bh_lock_sock().  However, bh_lock_sock()
does not provide mutual exclusion against process-context lock_sock()
held by nfc_llcp_accept_dequeue() during accept().  Since socket_release()
runs in process context (workqueue or kref cleanup), it never checks
sock_owned_by_user(), allowing both paths to manipulate the same child
socket concurrently.  This leads to a double sock_put() (Use-After-Free)
or a NULL pointer dereference of child->parent in nfc_llcp_accept_unlink().

Fix this by converting nfc_llcp_socket_release() to use lock_sock().
Since it is called with the local sockets write-lock held (which disables
preemption, preventing sleep), repeatedly pop the head socket from the
list under the write-lock using a new helper nfc_llcp_sock_list_pop(),
then safely acquire lock_sock() on it after dropping the list lock.
A sock_hold() is taken under the list lock to keep the socket alive
across the sleep and balanced with a sock_put() after release.

Additionally, add a check for a non-NULL parent in nfc_llcp_accept_unlink()
to ensure idempotency and prevent NULL pointer dereference if the unlink
races with dequeue.

Fixes: 50b78b2a6500 ("NFC: Fix sleeping in atomic when releasing socket")
Signed-off-by: Lee Jones <lee@kernel.org>
---
 net/nfc/llcp_core.c | 47 +++++++++++++++++++++++++--------------------
 net/nfc/llcp_sock.c | 13 +++++++------
 2 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index dc65c719f35f2..e51bd0d081b62 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -63,21 +63,33 @@ static void nfc_llcp_socket_purge(struct nfc_llcp_sock *sock)
 	}
 }
 
+static struct sock *nfc_llcp_sock_list_pop(struct llcp_sock_list *l)
+{
+	struct sock *sk;
+
+	write_lock(&l->lock);
+	sk = sk_head(&l->head);
+	if (sk) {
+		sock_hold(sk);
+		sk_del_node_init(sk);
+	}
+	write_unlock(&l->lock);
+
+	return sk;
+}
+
 static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 				    int err)
 {
 	struct sock *sk;
-	struct hlist_node *tmp;
 	struct nfc_llcp_sock *llcp_sock;
 
 	skb_queue_purge(&local->tx_queue);
 
-	write_lock(&local->sockets.lock);
-
-	sk_for_each_safe(sk, tmp, &local->sockets.head) {
+	while ((sk = nfc_llcp_sock_list_pop(&local->sockets))) {
 		llcp_sock = nfc_llcp_sock(sk);
 
-		bh_lock_sock(sk);
+		lock_sock(sk);
 
 		nfc_llcp_socket_purge(llcp_sock);
 
@@ -92,7 +104,8 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 						 &llcp_sock->accept_queue,
 						 accept_queue) {
 				accept_sk = &lsk->sk;
-				bh_lock_sock(accept_sk);
+				lock_sock_nested(accept_sk,
+						 SINGLE_DEPTH_NESTING);
 
 				nfc_llcp_accept_unlink(accept_sk);
 
@@ -101,7 +114,7 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 				accept_sk->sk_state = LLCP_CLOSED;
 				accept_sk->sk_state_change(sk);
 
-				bh_unlock_sock(accept_sk);
+				release_sock(accept_sk);
 			}
 		}
 
@@ -110,23 +123,18 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 		sk->sk_state = LLCP_CLOSED;
 		sk->sk_state_change(sk);
 
-		bh_unlock_sock(sk);
-
-		sk_del_node_init(sk);
+		release_sock(sk);
+		sock_put(sk);
 	}
 
-	write_unlock(&local->sockets.lock);
-
 	/* If we still have a device, we keep the RAW sockets alive */
 	if (device == true)
 		return;
 
-	write_lock(&local->raw_sockets.lock);
-
-	sk_for_each_safe(sk, tmp, &local->raw_sockets.head) {
+	while ((sk = nfc_llcp_sock_list_pop(&local->raw_sockets))) {
 		llcp_sock = nfc_llcp_sock(sk);
 
-		bh_lock_sock(sk);
+		lock_sock(sk);
 
 		nfc_llcp_socket_purge(llcp_sock);
 
@@ -135,12 +143,9 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 		sk->sk_state = LLCP_CLOSED;
 		sk->sk_state_change(sk);
 
-		bh_unlock_sock(sk);
-
-		sk_del_node_init(sk);
+		release_sock(sk);
+		sock_put(sk);
 	}
-
-	write_unlock(&local->raw_sockets.lock);
 }
 
 static struct nfc_llcp_local *nfc_llcp_local_get(struct nfc_llcp_local *local)
diff --git a/net/nfc/llcp_sock.c b/net/nfc/llcp_sock.c
index feab29fc62f44..3d30f0233b986 100644
--- a/net/nfc/llcp_sock.c
+++ b/net/nfc/llcp_sock.c
@@ -384,11 +384,12 @@ void nfc_llcp_accept_unlink(struct sock *sk)
 
 	pr_debug("state %d\n", sk->sk_state);
 
-	list_del_init(&llcp_sock->accept_queue);
-	sk_acceptq_removed(llcp_sock->parent);
-	llcp_sock->parent = NULL;
-
-	sock_put(sk);
+	if (llcp_sock->parent) {
+		list_del_init(&llcp_sock->accept_queue);
+		sk_acceptq_removed(llcp_sock->parent);
+		llcp_sock->parent = NULL;
+		sock_put(sk);
+	}
 }
 
 void nfc_llcp_accept_enqueue(struct sock *parent, struct sock *sk)
@@ -622,7 +623,7 @@ static int llcp_sock_release(struct socket *sock)
 		list_for_each_entry_safe(lsk, n, &llcp_sock->accept_queue,
 					 accept_queue) {
 			accept_sk = &lsk->sk;
-			lock_sock(accept_sk);
+			lock_sock_nested(accept_sk, SINGLE_DEPTH_NESTING);
 
 			nfc_llcp_send_disconnect(lsk);
 			nfc_llcp_accept_unlink(accept_sk);
-- 
2.54.0.823.g6e5bcc1fc9-goog