[PATCH net] nfc: llcp: Fix list corruption / refcount desync in nfc_llcp_recv_dm()

[Aldo Ariel Panzardo <qwe.aldo@gmail.com>]

nfc_llcp_recv_dm() handles DM(NOBOUND)/DM(REJ) for a socket that is still
linked on local->connecting_sockets: it looks the socket up with
nfc_llcp_connecting_sock_get(), sets sk->sk_state = LLCP_CLOSED and
returns, without taking the socket lock and without unlinking the socket
from the connecting_sockets list.

llcp_sock_release() selects the list to unlink from by sk_state: a socket
in LLCP_CONNECTING is unlinked from connecting_sockets, otherwise from the
sockets list.  Because recv_dm left the socket physically on
connecting_sockets but in the LLCP_CLOSED state, release() takes the else
branch and calls nfc_llcp_sock_unlink(&local->sockets, sk).  That runs
sk_del_node_init() while holding sockets.lock, i.e. it removes the socket
from the connecting_sockets hlist under the wrong lock.  A concurrent
connect() linking another socket onto connecting_sockets under
connecting_sockets.lock then mutates the same hlist unserialized, which
corrupts the list and desyncs the sk_add_node()/sk_del_node_init()
sock_hold()/__sock_put() pairing.  An unprivileged local process holding
LLCP sockets, with the DM supplied by the remote peer over an established
LLCP link, can drive this to leak kernel sockets without bound (the
mis-decrement goes through the non-freeing __sock_put() path, so the
object is never released), leading to memory exhaustion / DoS.

This is the same class of bug that was fixed in the sibling handler
nfc_llcp_recv_cc() by commit b493ea2765cc ("nfc: llcp: Fix use-after-free
race in nfc_llcp_recv_cc()"); recv_dm did not receive the equivalent fix.

Fix it the same way: take lock_sock(), re-check that the socket is still
hashed (release() may have won the race), and for the NOBOUND/REJ case
unlink it from connecting_sockets before moving it to LLCP_CLOSED.  The
unlink drops the connecting_sockets membership reference via
sk_del_node_init(), leaving the socket unhashed, so the later
nfc_llcp_sock_unlink() in llcp_sock_release() becomes a no-op and no
double put occurs.

Fixes: a69f32af86e3 ("NFC: Socket linked list")
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
---
 net/nfc/llcp_core.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index dc65c719f35f..d8dbb1bb857b 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1249,6 +1249,7 @@ static void nfc_llcp_recv_dm(struct nfc_llcp_local *local,
 	struct nfc_llcp_sock *llcp_sock;
 	struct sock *sk;
 	u8 dsap, ssap, reason;
+	bool connecting = false;
 
 	dsap = nfc_llcp_dsap(skb);
 	ssap = nfc_llcp_ssap(skb);
@@ -1260,6 +1261,7 @@ static void nfc_llcp_recv_dm(struct nfc_llcp_local *local,
 	case LLCP_DM_NOBOUND:
 	case LLCP_DM_REJ:
 		llcp_sock = nfc_llcp_connecting_sock_get(local, dsap);
+		connecting = true;
 		break;
 
 	default:
@@ -1274,10 +1276,33 @@ static void nfc_llcp_recv_dm(struct nfc_llcp_local *local,
 
 	sk = &llcp_sock->sk;
 
+	lock_sock(sk);
+
+	/* Check if socket was destroyed whilst waiting for the lock */
+	if (!sk_hashed(sk)) {
+		release_sock(sk);
+		nfc_llcp_sock_put(llcp_sock);
+		return;
+	}
+
+	/*
+	 * For DM(NOBOUND)/DM(REJ) the socket is still linked on the
+	 * connecting_sockets list.  Unlink it here, under the socket lock,
+	 * before moving it to LLCP_CLOSED: llcp_sock_release() selects the
+	 * list to unlink from by sk_state, so leaving a connecting socket
+	 * in the CLOSED state would make it unlink from the wrong list and
+	 * corrupt the connecting_sockets list / desync the socket refcount.
+	 * This mirrors nfc_llcp_recv_cc().
+	 */
+	if (connecting)
+		nfc_llcp_sock_unlink(&local->connecting_sockets, sk);
+
 	sk->sk_err = ENXIO;
 	sk->sk_state = LLCP_CLOSED;
 	sk->sk_state_change(sk);
 
+	release_sock(sk);
+
 	nfc_llcp_sock_put(llcp_sock);
 }
 

base-commit: 3f1f755366687d051174739fb99f7d560202f60b
-- 
2.43.0