On Wed, Dec 6, 2023 at 9:03 PM Heng Qi [off-list ref] wrote:

在 2023/12/6 下午8:27, Paolo Abeni 写道:

quoted

On Tue, 2023-12-05 at 19:05 +0800, Heng Qi wrote:

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在 2023/12/5 下午4:35, Jason Wang 写道:

quoted

On Tue, Dec 5, 2023 at 4:02 PM Heng Qi [off-list ref] wrote:

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Currently access to ctrl cmd is globally protected via rtnl_lock and works
fine. But if dim work's access to ctrl cmd also holds rtnl_lock, deadlock
may occur due to cancel_work_sync for dim work.

Can you explain why?

For example, during the bus unbind operation, the following call stack
occurs:
virtnet_remove -> unregister_netdev -> rtnl_lock[1] -> virtnet_close ->
cancel_work_sync -> virtnet_rx_dim_work -> rtnl_lock[2] (deadlock occurs).

quoted

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Therefore, treating
ctrl cmd as a separate protection object of the lock is the solution and
the basis for the next patch.

Let's don't do that. Reasons are:

1) virtnet_send_command() may wait for cvq commands for an indefinite time

Yes, I took that into consideration. But ndo_set_rx_mode's need for an
atomic
environment rules out the mutex lock.

quoted

2) hold locks may complicate the future hardening works around cvq

Agree, but I don't seem to have thought of a better way besides passing
the lock.
Do you have any other better ideas or suggestions?

What about:

- using the rtnl lock only
- virtionet_close() invokes cancel_work(), without flushing the work
- virtnet_remove() calls flush_work() after unregister_netdev(),
outside the rtnl lock

Should prevent both the deadlock and the UaF.

Hi, Paolo and Jason!

Thank you very much for your effective suggestions, but I found another
solution[1],
based on the ideas of rtnl_trylock and refill_work, which works very well:

[1]
+static void virtnet_rx_dim_work(struct work_struct *work)
+{
+    struct dim *dim = container_of(work, struct dim, work);
+    struct receive_queue *rq = container_of(dim,
+            struct receive_queue, dim);
+    struct virtnet_info *vi = rq->vq->vdev->priv;
+    struct net_device *dev = vi->dev;
+    struct dim_cq_moder update_moder;
+    int i, qnum, err;
+
+    if (!rtnl_trylock())
+        return;

Don't we need to reschedule here?

like

if (rq->dim_enabled)
       sechedule_work()

?

Thanks

+
+    for (i = 0; i < vi->curr_queue_pairs; i++) {
+        rq = &vi->rq[i];
+        dim = &rq->dim;
+        qnum = rq - vi->rq;
+
+        if (!rq->dim_enabled)
+            continue;
+
+        update_moder = net_dim_get_rx_moderation(dim->mode,
dim->profile_ix);
+        if (update_moder.usec != rq->intr_coal.max_usecs ||
+            update_moder.pkts != rq->intr_coal.max_packets) {
+            err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, qnum,
+                                   update_moder.usec,
+                                   update_moder.pkts);
+            if (err)
+                pr_debug("%s: Failed to send dim parameters on rxq%d\n",
+                     dev->name, qnum);
+            dim->state = DIM_START_MEASURE;
+        }
+    }
+
+    rtnl_unlock();
+}


In addition, other optimizations[2] have been tried, but it may be due
to the sparsely
scheduled work that the retry condition is always satisfied, affecting
performance,
so [1] is the final solution:

[2]

+static void virtnet_rx_dim_work(struct work_struct *work)
+{
+    struct dim *dim = container_of(work, struct dim, work);
+    struct receive_queue *rq = container_of(dim,
+            struct receive_queue, dim);
+    struct virtnet_info *vi = rq->vq->vdev->priv;
+    struct net_device *dev = vi->dev;
+    struct dim_cq_moder update_moder;
+    int i, qnum, err, count;
+
+    if (!rtnl_trylock())
+        return;
+retry:
+    count = vi->curr_queue_pairs;
+    for (i = 0; i < vi->curr_queue_pairs; i++) {
+        rq = &vi->rq[i];
+        dim = &rq->dim;
+        qnum = rq - vi->rq;
+        update_moder = net_dim_get_rx_moderation(dim->mode,
dim->profile_ix);
+        if (update_moder.usec != rq->intr_coal.max_usecs ||
+            update_moder.pkts != rq->intr_coal.max_packets) {
+            --count;
+            err = virtnet_send_rx_ctrl_coal_vq_cmd(vi, qnum,
+                                   update_moder.usec,
+                                   update_moder.pkts);
+            if (err)
+                pr_debug("%s: Failed to send dim parameters on rxq%d\n",
+                     dev->name, qnum);
+            dim->state = DIM_START_MEASURE;
+        }
+    }
+
+    if (need_resched()) {
+        rtnl_unlock();
+        schedule();
+    }
+
+    if (count)
+        goto retry;
+
+    rtnl_unlock();
+}

Thanks a lot!

quoted

Side note: for this specific case any functional test with a
CONFIG_LOCKDEP enabled build should suffice to catch the deadlock
scenario above.

Cheers,

Paolo