Hi Desmond, On Thu, Sep 2, 2021 at 4:05 PM Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> wrote: > > On 2/9/21 6:53 pm, Desmond Cheong Zhi Xi wrote: > > On 2/9/21 5:41 pm, Eric Dumazet wrote: > >> > >> > >> 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@xxxxxxxxx/ > >>> > >> > >> > >> 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; > >> > > > > I see where you're going with this, but once sco_chan_del returns, any > > instance of sco_sock_timeout that hasn't yet called sock_hold will > > simply return, because conn->sk is NULL. Adding a delay to the > > sco_conn_lock critical section in sco_sock_timeout would not affect this > > because sco_chan_del clears conn->sk while holding onto the lock. > > > > The main reason that cancel_delayed_work_sync is run there is to make > > sure that we don't have a UAF on the SCO connection itself after we free > > conn. > > > > Now that I think about this, the init and cleanup isn't quite right > either. The delayed work should be initialized when the connection is > allocated, and we should always cancel all work before freeing: > > diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c > index ea18e5b56343..bba5cdb4cb4a 100644 > --- a/net/bluetooth/sco.c > +++ b/net/bluetooth/sco.c > @@ -133,6 +133,7 @@ static struct sco_conn *sco_conn_add(struct hci_conn *hcon) > return NULL; > > spin_lock_init(&conn->lock); > + INIT_DELAYED_WORK(&conn->timeout_work, sco_sock_timeout); > > hcon->sco_data = conn; > conn->hcon = hcon; > @@ -197,11 +198,11 @@ static void sco_conn_del(struct hci_conn *hcon, int err) > 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); > } > > + /* Ensure no more work items will run before freeing conn. */ > + cancel_delayed_work_sync(&conn->timeout_work); > + > hcon->sco_data = NULL; > kfree(conn); > } > @@ -214,8 +215,6 @@ static void __sco_chan_add(struct sco_conn *conn, struct sock *sk, > sco_pi(sk)->conn = conn; > conn->sk = sk; > > - INIT_DELAYED_WORK(&conn->timeout_work, sco_sock_timeout); > - > if (parent) > bt_accept_enqueue(parent, sk, true); > } I have come to something similar, do you care to send a proper patch so we can get this merged. > > For a single SCO connection with well-formed channel, I think there > > can't be a race. Here's the reasoning: > > > > - For the timeout to be scheduled, a socket must have a channel with a > > connection. > > > > - When a channel between a socket and connection is established, the > > socket transitions from BT_OPEN to BT_CONNECTED, BT_CONNECT, or > > BT_CONNECT2. > > > > - For a socket to be released, it has to be zapped. For sockets that > > have a state of BT_CONNECTED, BT_CONNECT, or BT_CONNECT2, they are > > zapped only when the channel is deleted. > > > > - If the channel is deleted (which is protected by sco_conn_lock), then > > conn->sk is NULL, and sco_sock_timeout simply exits. If we had entered > > the critical section in sco_sock_timeout before the channel was deleted, > > then we increased the reference count on the socket, so it won't be > > freed until sco_sock_timeout is done. > > > > Hence, sco_sock_timeout doesn't race with the release of a socket that > > has a well-formed channel with a connection. > > > > But if multiple connections are allocated and overwritten in > > sco_sock_connect, then none of the above assumptions hold because the > > SCO connection can't be cleaned up (i.e. conn->sk cannot be set to NULL) > > when the associated socket is released. This scenario happens in the > > syzbot reproducer for the crash here: > > https://syzkaller.appspot.com/bug?id=bcc246d137428d00ed14b476c2068579515fe2bc > > > > > > That aside, upon taking a closer look, I think there is indeed a race > > lurking in sco_conn_del, but it's not the one that syzbot is hitting. > > Our sock_hold simply comes too late, and by the time it's called we > > might have already have freed the socket. > > > > So probably something like this needs to happen: > > > > diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c > > index fa25b07120c9..ea18e5b56343 100644 > > --- a/net/bluetooth/sco.c > > +++ b/net/bluetooth/sco.c > > @@ -187,10 +187,11 @@ static void sco_conn_del(struct hci_conn *hcon, > > int err) > > /* Kill socket */ > > sco_conn_lock(conn); > > sk = conn->sk; > > + if (sk) > > + sock_hold(sk); > > sco_conn_unlock(conn); > > > > if (sk) { > > - sock_hold(sk); > > lock_sock(sk); > > sco_sock_clear_timer(sk); > > sco_chan_del(sk, err); > -- Luiz Augusto von Dentz
