[PATCH net-next v5 1/2] nfc: llcp_core: Hold a ref to llcp_local->dev when holding a ref to llcp_local

[Siddh Raman Pant]

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.

Reported-and-tested-by: syzbot+bbe84a4010eeea00982d@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=bbe84a4010eeea00982d
Fixes: c7aa12252f51 ("NFC: Take a reference on the LLCP local pointer when creating a socket")
Reviewed-by: Suman Ghosh <sumang@xxxxxxxxxxx>
Signed-off-by: Siddh Raman Pant <code@xxxxxxxx>
---
 net/nfc/llcp_core.c | 72 +++++++++++++++++++++++++++++----------------
 1 file changed, 47 insertions(+), 25 deletions(-)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index 1dac28136e6a..2f77200a3720 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -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,
@@ -901,7 +916,7 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 
 	if (dsap != LLCP_SAP_SDP) {
 		sock = nfc_llcp_sock_get(local, dsap, LLCP_SAP_SDP);
-		if (sock == NULL || sock->sk.sk_state != LLCP_LISTEN) {
+		if (!sock || sock->sk.sk_state != LLCP_LISTEN) {
 			reason = LLCP_DM_NOBOUND;
 			goto fail;
 		}
@@ -910,7 +925,7 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 		size_t sn_len;
 
 		sn = nfc_llcp_connect_sn(skb, &sn_len);
-		if (sn == NULL) {
+		if (!sn) {
 			reason = LLCP_DM_NOBOUND;
 			goto fail;
 		}
@@ -918,7 +933,7 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 		pr_debug("Service name length %zu\n", sn_len);
 
 		sock = nfc_llcp_sock_get_sn(local, sn, sn_len);
-		if (sock == NULL) {
+		if (!sock) {
 			reason = LLCP_DM_NOBOUND;
 			goto fail;
 		}
@@ -928,39 +943,31 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 
 	parent = &sock->sk;
 
-	if (sk_acceptq_is_full(parent)) {
-		reason = LLCP_DM_REJ;
-		release_sock(&sock->sk);
-		sock_put(&sock->sk);
-		goto fail;
-	}
+	if (sk_acceptq_is_full(parent))
+		goto fail_put_sock;
 
 	if (sock->ssap == LLCP_SDP_UNBOUND) {
 		u8 ssap = nfc_llcp_reserve_sdp_ssap(local);
 
 		pr_debug("First client, reserving %d\n", ssap);
 
-		if (ssap == LLCP_SAP_MAX) {
-			reason = LLCP_DM_REJ;
-			release_sock(&sock->sk);
-			sock_put(&sock->sk);
-			goto fail;
-		}
+		if (ssap == LLCP_SAP_MAX)
+			goto fail_put_sock;
 
 		sock->ssap = ssap;
 	}
 
 	new_sk = nfc_llcp_sock_alloc(NULL, parent->sk_type, GFP_ATOMIC, 0);
-	if (new_sk == NULL) {
-		reason = LLCP_DM_REJ;
-		release_sock(&sock->sk);
-		sock_put(&sock->sk);
-		goto fail;
-	}
+	if (!new_sk)
+		goto fail_put_sock;
 
 	new_sock = nfc_llcp_sock(new_sk);
-	new_sock->dev = local->dev;
+
 	new_sock->local = nfc_llcp_local_get(local);
+	if (!new_sock->local)
+		goto fail_free_new_sock;
+
+	new_sock->dev = local->dev;
 	new_sock->rw = sock->rw;
 	new_sock->miux = sock->miux;
 	new_sock->nfc_protocol = sock->nfc_protocol;
@@ -1004,8 +1011,14 @@ static void nfc_llcp_recv_connect(struct nfc_llcp_local *local,
 
 	return;
 
+fail_free_new_sock:
+	sock_put(&new_sock->sk);
+	nfc_llcp_sock_free(new_sock);
+fail_put_sock:
+	reason = LLCP_DM_REJ;
+	release_sock(&sock->sk);
+	sock_put(&sock->sk);
 fail:
-	/* Send DM */
 	nfc_llcp_send_dm(local, dsap, ssap, reason);
 }
 
@@ -1597,7 +1610,16 @@ 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