This is a note to let you know that I've just added the patch titled
Bluetooth: Avoid bt_accept_unlink() double unlinking
to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: bluetooth-avoid-bt_accept_unlink-double-unlinking.patch and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From foo@baz Sun Mar 18 16:55:33 CET 2018
From: Dean Jenkins Dean_Jenkins@mentor.com Date: Fri, 10 Mar 2017 11:34:46 +0000 Subject: Bluetooth: Avoid bt_accept_unlink() double unlinking
From: Dean Jenkins Dean_Jenkins@mentor.com
[ Upstream commit 27bfbc21a0c0f711fa5382de026c7c0700c9ea28 ]
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 Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/bluetooth/af_bluetooth.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+)
--- a/net/bluetooth/af_bluetooth.c +++ b/net/bluetooth/af_bluetooth.c @@ -163,6 +163,9 @@ void bt_accept_enqueue(struct sock *pare } EXPORT_SYMBOL(bt_accept_enqueue);
+/* Calling function must hold the sk lock. + * bt_sk(sk)->parent must be non-NULL meaning sk is in the parent list. + */ void bt_accept_unlink(struct sock *sk) { BT_DBG("sk %p state %d", sk, sk->sk_state); @@ -181,11 +184,32 @@ struct sock *bt_accept_dequeue(struct so
BT_DBG("parent %p", parent);
+restart: list_for_each_entry_safe(s, n, &bt_sk(parent)->accept_q, accept_q) { sk = (struct sock *)s;
+ /* Prevent early freeing of sk due to unlink and sock_kill */ + sock_hold(sk); lock_sock(sk);
+ /* Check sk has not already been unlinked via + * bt_accept_unlink() due to serialisation caused by sk locking + */ + if (!bt_sk(sk)->parent) { + BT_DBG("sk %p, already unlinked", sk); + release_sock(sk); + sock_put(sk); + + /* Restart the loop as sk is no longer in the list + * and also avoid a potential infinite loop because + * list_for_each_entry_safe() is not thread safe. + */ + goto restart; + } + + /* sk is safely in the parent list so reduce reference count */ + sock_put(sk); + /* FIXME: Is this check still needed */ if (sk->sk_state == BT_CLOSED) { bt_accept_unlink(sk);
Patches currently in stable-queue which might be from Dean_Jenkins@mentor.com are
queue-4.9/bluetooth-avoid-bt_accept_unlink-double-unlinking.patch