In the Linux kernel, the following vulnerability has been resolved: nfc: llcp: Fix list corruption / refcount desync in nfc_llcp_recv_dm()

In the Linux kernel, the following vulnerability has been resolved:

nfc: llcp: Fix list corruption / refcount desync in nfc_llcp_recv_dm()

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.