Bluetooth: Avoid bt_accept_unlink() double unlinking

There is a race condition between a thread calling bt_accept_dequeue()
and a different thread calling bt_accept_unlink(). Protection against
concurrency is implemented using sk locking. However, sk locking causes
serialisation of the bt_accept_dequeue() and bt_accept_unlink() threads.
This serialisation can cause bt_accept_dequeue() to obtain the sk from the
parent list but becomes blocked waiting for the sk lock held by the
bt_accept_unlink() thread. bt_accept_unlink() unlinks sk and this thread
releases the sk lock unblocking bt_accept_dequeue() which potentially runs
bt_accept_unlink() again on the same sk causing a crash. The attempt to
double unlink the same sk from the parent list can cause a NULL pointer
dereference crash due to bt_sk(sk)->parent becoming NULL on the first
unlink, followed by the second unlink trying to execute
bt_sk(sk)->parent->sk_ack_backlog-- in bt_accept_unlink() which crashes.

When sk is in the parent list, bt_sk(sk)->parent will be not be NULL.
When sk is removed from the parent list, bt_sk(sk)->parent is set to
NULL. Therefore, add a defensive check for bt_sk(sk)->parent not being
NULL to ensure that sk is still in the parent list after the sk lock has
been taken in bt_accept_dequeue(). If bt_sk(sk)->parent is detected as
being NULL then restart the loop so that the loop variables are refreshed
to use the latest values. This is necessary as list_for_each_entry_safe()
is not thread safe so causing a risk of an infinite loop occurring as sk
could point to itself.

In addition, in bt_accept_dequeue() increase the sk reference count to
protect against early freeing of sk. Early freeing can be possible if the
bt_accept_unlink() thread calls l2cap_sock_kill() or rfcomm_sock_kill()
functions before bt_accept_dequeue() gets the sk lock.

For test purposes, the probability of failure can be increased by putting
a msleep of 1 second in bt_accept_dequeue() between getting the sk and
waiting for the sk lock. This exposes the fact that the loop
list_for_each_entry_safe(p, n, &bt_sk(parent)->accept_q) is not safe from
threads that unlink sk from the list in parallel with the loop which can
cause sk to become stale within the loop.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>