This patch addresses a socket state validation gap in NFC LLCP socket
connection logic where connect() can be invoked on sockets in the
LLCP_BOUND (or LLCP_LISTEN) state.

Historical background and context:
Earlier patches addressing CVE-2020-25670 / CVE-2020-25673 added an
explicit check for the LLCP_CONNECTING state in llcp_sock_connect() to
prevent duplicate in-flight connection requests on non-blocking sockets
from linking the socket multiple times into local->connecting_sockets.

However, that check implicitly assumed that any socket that was neither
LLCP_CONNECTED nor LLCP_CONNECTING was in LLCP_CLOSED. It omitted the
possibility that a socket had already been bound via llcp_sock_bind(),
which transitions the socket to LLCP_BOUND and adds it to local->sockets.

Vulnerability impact:
When connect() is called on an already bound socket:
1. llcp_sock_connect() overwrites llcp_sock->dev and llcp_sock->local
   without dropping the original references, causing refcount and memory
   leaks (service_name).
2. The socket is added to local->connecting_sockets while still remaining
   linked in local->sockets.
3. Upon receiving connection confirmation (CC), nfc_llcp_recv_cc() unlinks
   the socket from connecting_sockets and links it into local->sockets.
   Because the head of local->sockets still pointed to the socket itself,
   hlist_add_head() links the node to itself (node->next = node), forming
   an infinite loop. Subsequent traversals of local->sockets (e.g. during
   lookup or release) loop indefinitely under spinlock, leading to kernel
   soft-lockups and RCU stalls.
4. If connect() targets a different device index, the socket becomes linked
   across two different NFC devices, causing cross-device socket confusion.

Fix:
Enforce that sockets must be in LLCP_CLOSED before initiating a connection,
returning -EBADFD otherwise, aligning with the state check behavior in
llcp_sock_bind() and llcp_raw_sock_bind().

Tested and confirmed style-compliant with checkpatch.pl
(0 errors, 0 warnings).

Yuchao Zhang (1):
  nfc: llcp: reject connect() on sockets not in LLCP_CLOSED state

 net/nfc/llcp_sock.c | 4 ++++
 1 file changed, 4 insertions(+)

-- 
2.53.0