On 9/2/21 12:32 PM, Desmond Cheong Zhi Xi wrote:

Hi Eric,

This actually seems to be a pre-existing error in sco_sock_connect that we now hit in sco_sock_timeout.

Any thoughts on the following patch to address the problem?

Link: https://lore.kernel.org/lkml/20210831065601.101185-1-desmondcheongzx@gmail.com/ (local)

syzbot is still working on finding a repro, this is obviously not trivial,
because this is a race window.

I think this can happen even with a single SCO connection.

This might be triggered more easily forcing a delay in sco_sock_timeout()

diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
index 98a88158651281c9f75c4e0371044251e976e7ef..71ebe0243fab106c676c308724fe3a3f92a62cbd 100644
--- a/net/bluetooth/sco.c
+++ b/net/bluetooth/sco.c

@@ -84,8 +84,14 @@ static void sco_sock_timeout(struct work_struct *work)
 
        sco_conn_lock(conn);
        sk = conn->sk;
-       if (sk)
+       if (sk) {
+               // lets pretend cpu has been busy (in interrupts) for 100ms
+               int i;
+               for (i=0;i<100000;i++)
+                       udelay(1);
+
                sock_hold(sk);
+       }
        sco_conn_unlock(conn);
 
        if (!sk)

Stack trace tells us that sco_sock_timeout() is running after last reference
on socket has been released.

__refcount_add include/linux/refcount.h:199 [inline]
 __refcount_inc include/linux/refcount.h:250 [inline]
 refcount_inc include/linux/refcount.h:267 [inline]
 sock_hold include/net/sock.h:702 [inline]
 sco_sock_timeout+0x216/0x290 net/bluetooth/sco.c:88
 process_one_work+0x98d/0x1630 kernel/workqueue.c:2276
 worker_thread+0x658/0x11f0 kernel/workqueue.c:2422
 kthread+0x3e5/0x4d0 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

This is why I suggested to delay sock_put() to make sure this can not happen.

diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
index 98a88158651281c9f75c4e0371044251e976e7ef..bd0222e3f05a6bcb40cffe8405c9dfff98d7afde 100644
--- a/net/bluetooth/sco.c
+++ b/net/bluetooth/sco.c

@@ -195,10 +195,11 @@ static void sco_conn_del(struct hci_conn *hcon, int err)
                sco_sock_clear_timer(sk);
                sco_chan_del(sk, err);
                release_sock(sk);
-               sock_put(sk);
 
                /* Ensure no more work items will run before freeing conn. */
                cancel_delayed_work_sync(&conn->timeout_work);
+
+               sock_put(sk);
        }
 
        hcon->sco_data = NULL;