nfc: Fix UAF during datagram sending caused by missing refcounting

Changes in v6:
- Revert label introduction from v4, and thus also v5 entirely.

Changes in v5:
- Move reason = LLCP_DM_REJ under the fail_put_sock label.
- Checkpatch now warns about == NULL check for new_sk, so fix that, and also at other similar places in the same function.

Changes in v4:
- Fix put ordering and comments.
- Separate freeing in recv() into end labels.
- Remove obvious comment and add reasoning.
- Picked up r-bs by Suman.

Changes in v3:
- Fix missing freeing statements.

Changes in v2:
- Add net-next in patch subject.
- Removed unnecessary extra lock and hold nfc_dev ref when holding llcp_sock.
- Remove last formatting patch.
- Picked up r-b from Krzysztof for LLCP_BOUND patch.

---

For connectionless transmission, llcp_sock_sendmsg() codepath will eventually call nfc_alloc_send_skb() which takes in an nfc_dev as an argument for calculating the total size for skb allocation.

virtual_ncidev_close() codepath eventually releases socket by calling nfc_llcp_socket_release() (which sets the sk->sk_state to LLCP_CLOSED) and afterwards the nfc_dev will be eventually freed.

When an ndev gets freed, llcp_sock_sendmsg() will result in an use-after-free as it

(1) doesn't have any checks in place for avoiding the datagram sending.

(2) calls nfc_llcp_send_ui_frame(), which also has a do-while loop which can race with freeing. This loop contains the call to nfc_alloc_send_skb() where we dereference the nfc_dev pointer.

nfc_dev is being freed because we do not hold a reference to it when we hold a reference to llcp_local. Thus, virtual_ncidev_close() eventually calls nfc_release() due to refcount going to 0.

Since state has to be LLCP_BOUND for datagram sending, we can bail out early in llcp_sock_sendmsg().

Please review and let me know if any errors are there, and hopefully this gets accepted.

Thanks,
Siddh

Siddh Raman Pant (2):
nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to
llcp_local
nfc: Do not send datagram if socket state isn't LLCP_BOUND

 net/nfc/llcp_core.c | 40 +++++++++++++++++++++++++++++++++++++---
 net/nfc/llcp_sock.c |  5 +++++
 2 files changed, 42 insertions(+), 3 deletions(-)

2.42.0

llcp_sock_sendmsg() calls nfc_llcp_send_ui_frame() which in turn calls nfc_alloc_send_skb(), which accesses the nfc_dev from the llcp_sock for getting the headroom and tailroom needed for skb allocation.

Parallelly the nfc_dev can be freed, as the refcount is decreased via nfc_free_device(), leading to a UAF reported by Syzkaller, which can be summarized as follows:

(1) llcp_sock_sendmsg() -> nfc_llcp_send_ui_frame()

	-> nfc_alloc_send_skb() -> Dereference *nfc_dev
(2) virtual_ncidev_close() -> nci_free_device() -> nfc_free_device()
	-> put_device() -> nfc_release() -> Free *nfc_dev

When a reference to llcp_local is acquired, we do not acquire the same for the nfc_dev. This leads to freeing even when the llcp_local is in use, and this is the case with the UAF described above too.

Thus, when we acquire a reference to llcp_local, we should acquire a reference to nfc_dev, and release the references appropriately later.

References for llcp_local is initialized in nfc_llcp_register_device() (which is called by nfc_register_device()). Thus, we should acquire a reference to nfc_dev there.

nfc_unregister_device() calls nfc_llcp_unregister_device() which in turn calls nfc_llcp_local_put(). Thus, the reference to nfc_dev is appropriately released later.

---
 net/nfc/llcp_core.c | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

index 1dac28136e6a..fadc8a9ec4df 100644
@@ -145,6 +145,13 @@ static void nfc_llcp_socket_release(struct nfc_llcp_local *local, bool device,
 
 static struct nfc_llcp_local *nfc_llcp_local_get(struct nfc_llcp_local *local)
 {
+	/* Since using nfc_llcp_local may result in usage of nfc_dev, whenever
+	 * we hold a reference to local, we also need to hold a reference to
+	 * the device to avoid UAF.
+	 */
+	if (!nfc_get_device(local->dev->idx))
+		return NULL;
+
 	kref_get(&local->ref);
 
 	return local;

@@ -177,10 +184,18 @@ static void local_release(struct kref *ref)
 
 int nfc_llcp_local_put(struct nfc_llcp_local *local)
 {
+	struct nfc_dev *dev;
+	int ret;
+
 	if (local == NULL)
 		return 0;
 
-	return kref_put(&local->ref, local_release);
+	dev = local->dev;
+
+	ret = kref_put(&local->ref, local_release);
+	nfc_put_device(dev);
+
+	return ret;
 }
 
 static struct nfc_llcp_sock *nfc_llcp_sock_get(struct nfc_llcp_local *local,

@@ -959,8 +974,18 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 	}
 
 	new_sock = nfc_llcp_sock(new_sk);
-	new_sock->dev = local->dev;
+
 	new_sock->local = nfc_llcp_local_get(local);
+	if (!new_sock->local) {
+		reason = LLCP_DM_REJ;
+		release_sock(&sock->sk);
+		sock_put(&sock->sk);
+		sock_put(&new_sock->sk);
+		nfc_llcp_sock_free(new_sock);
+		goto fail;
+	}
+
+	new_sock->dev = local->dev;
 	new_sock->rw = sock->rw;
 	new_sock->miux = sock->miux;
 	new_sock->nfc_protocol = sock->nfc_protocol;

@@ -1597,7 +1622,16 @@ int nfc_llcp_register_device(struct nfc_dev *ndev)
 	if (local == NULL)
 		return -ENOMEM;
 
-	local->dev = ndev;
+	/* As we are going to initialize local's refcount, we need to get the
+	 * nfc_dev to avoid UAF, otherwise there is no point in continuing.
+	 * See nfc_llcp_local_get().
+	 */
+	local->dev = nfc_get_device(ndev->idx);
+	if (!local->dev) {
+		kfree(local);
+		return -ENODEV;
+	}
+
 	INIT_LIST_HEAD(&local->list);
 	kref_init(&local->ref);
 	mutex_init(&local->sdp_lock);

2.42.0

Thanks for pointing out. sk_init sets refcount to 1. Actually on a further look, the next line shouldn't be there as nfc_llcp_sock_free() is already called in sk->sk_destruct (== llcp_sock_destruct()), which is called via __sk_destruct().

As sock_put() -> sk_free() -> __sk_destruct() -> sk_prot_free(), so we need to put.

TBH really don't know why nfc_llcp_sock_free() is not static.

Oops, extremely sorry about that :( I reverted back to wrong ordering from an older local commit and didn't check.

I'll send the fixed one.

Thanks,
Siddh

As we know we cannot send the datagram (state can be set to LLCP_CLOSED by nfc_llcp_socket_release()), there is no need to proceed further.

Thus, bail out early from llcp_sock_sendmsg().

---
 net/nfc/llcp_sock.c | 5 +++++
 1 file changed, 5 insertions(+)

index 645677f84dba..819157bbb5a2 100644
@@ -796,6 +796,11 @@ static int llcp_sock_sendmsg(struct socket *sock, struct msghdr *msg,
 	}
 
 	if (sk->sk_type == SOCK_DGRAM) {
+		if (sk->sk_state != LLCP_BOUND) {
+			release_sock(sk);
+			return -ENOTCONN;
+		}
+
 		DECLARE_SOCKADDR(struct sockaddr_nfc_llcp *, addr,
 				 msg->msg_name);
 

2.42.0