Re: Use-after-free access in j1939_session_deactivate

j1939_session_destroy() will free both session and session->priv. It
leads to multiple use-after-free read and write in
j1939_session_deactivate() when session was freed in
j1939_session_deactivate_locked(). The free chain is
j1939_session_deactivate_locked()->
j1939_session_put()->__j1939_session_release()->j1939_session_destroy().
To fix this bug, I moved j1939_session_put() behind
j1939_session_deactivate_locked() and guarded it with a check of
active since the session would be freed only if active is true.

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index e5f1a56994c6..b6448f29a4bd 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c

@@ -1018,7 +1018,6 @@ static bool

j1939_session_deactivate_locked(struct j1939_session *session)

        list_del_init(&session->active_session_list_entry);
        session->state = J1939_SESSION_DONE;
-       j1939_session_put(session);
    }

    return active;

@@ -1031,6 +1030,9 @@ static bool j1939_session_deactivate(struct

j1939_session *session)
    j1939_session_list_lock(session->priv);
    active = j1939_session_deactivate_locked(session);
    j1939_session_list_unlock(session->priv);
+   if (active) {
+       j1939_session_put(session);
+   }

    return active;
 }

@@ -2021,6 +2023,7 @@ void j1939_simple_recv(struct j1939_priv *priv,

struct sk_buff *skb)
 int j1939_cancel_active_session(struct j1939_priv *priv, struct sock *sk)
 {
    struct j1939_session *session, *saved;
+   bool active;

    netdev_dbg(priv->ndev, "%s, sk: %p\n", __func__, sk);
    j1939_session_list_lock(priv);

@@ -2030,7 +2033,10 @@ int j1939_cancel_active_session(struct

j1939_priv *priv, struct sock *sk)
        if (!sk || sk == session->sk) {
            j1939_session_timers_cancel(session);
            session->err = ESHUTDOWN;
-           j1939_session_deactivate_locked(session);
+           active = j1939_session_deactivate_locked(session);
+           if (active) {
+               j1939_session_put(session);
+           }
        }
    }
    j1939_session_list_unlock(priv);

On Mon, Jul 12, 2021 at 9:44 PM Greg KH [off-list ref] wrote:

On Mon, Jul 12, 2021 at 03:40:46PM -0700, Xiaochen Zou wrote:

quoted

Hi,
It looks like there are multiple use-after-free accesses in
j1939_session_deactivate()

static bool j1939_session_deactivate(struct j1939_session *session)
{
bool active;

j1939_session_list_lock(session->priv);
active = j1939_session_deactivate_locked(session); //session can be freed inside
j1939_session_list_unlock(session->priv); // It causes UAF read and write

return active;
}

session can be freed by
j1939_session_deactivate_locked->j1939_session_put->__j1939_session_release->j1939_session_destroy->kfree.
Therefore it makes the unlock function perform UAF access.

Great, can you make up a patch to fix this issue so you can get credit
for finding and solving it?

thanks,

greg k-h

-- 
Xiaochen Zou
PhD Student
Department of Computer Science & Engineering
University of California, Riverside