[PATCH 5.10.y 0/2] mptcp: fix ADD_ADDR retransmission socket leaks

Backport the two ordered ADD_ADDR retransmission timer fixes missing from
5.10.y. Patch 1 makes every terminal path release the socket reference
held by the timer. Patch 2 uses sock_put() so the last reference frees the
socket, while avoiding a timer self-deadlock during destruction.

Both fixes carry Cc: stable and are present in 5.15.y and every newer
supported LTS. The original 11-patch cover groups patches 3-5 as the
v5.10 ADD_ADDR timer data-race and leak fixes. Patch 3 is already present
as d9b272a85fe6 ("mptcp: pm: ADD_ADDR rtx: fix potential data-race");
this series supplies the missing ordered patches 4 and 5:
https://patch.msgid.link/20260505-net-mptcp-pm-fixes-7-1-rc3-v1-0-fca8091060a4@kernel.org

In v5.10 the timer code is in pm_netlink.c and uses fixed retransmission
delays; the adaptations retain those details. Patch 1 also routes v5.10's
unreachable ID 0 guard through the reference drop.

Tested on v5.10.269 with KASAN under QEMU. A targeted A/B forced the
tracked MPTCP socket to TCP_CLOSE at timer entry: the vulnerable kernel
ran the callback without destroying the socket, while the fixed kernel
destroyed it. Both completed an identical MPTCP transfer without a KASAN
report, Oops, or warning.

The fixed kernel also passed all 15 tests in pm_netlink.sh, a 16 MiB
bidirectional transfer with a real ADD_ADDR/MP_JOIN second subflow, and an
8 MiB curl --mptcp transfer. All payload comparisons were byte-exact.

Matthieu Baerts (NGI0) (2):
  mptcp: pm: ADD_ADDR rtx: always decrease sk refcount
  mptcp: pm: ADD_ADDR rtx: free sk if last

 net/mptcp/pm_netlink.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

-- 
2.53.0