[libvirt] [PATCH] remote_daemon_stream: Hold an extra reference to stream in daemonStreamFilter

[Michal Privoznik]

In v5.9.0-273-g8ecab214de I've tried to fix a lock ordering
problem, but introduced a crasher. Problem is that because the
client lock is unlocked (in order to honour lock ordering) the
stream we are currently checking in daemonStreamFilter() might be
freed and thus stream->priv might not even exist when the control
get to virMutexLock() call.

To resolve this, grab an extra reference to the stream and handle
its cleanup should the refcounter reach zero after the deref.
If that's the case and we are the only ones holding a reference
to the stream, we MUST return a positive value to make
virNetServerClientDispatchRead() break its loop where it iterates
over filters. The problem is, if we did not do so, then
"filter = filter->next" line will read from a memory that was
just freed (freeing a stream also unregisters its filter).

Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---

Reproducing this issue is very easy:

1) put sleep(5) right after virObjectUnlock(client) in the fist hunk,
2) virsh console --force $dom   and type something so that the stream
   has some data to process
3) while 2) is still running, run the same command from another terminal
4) observe libvirtd crash

 src/remote/remote_daemon_stream.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/remote/remote_daemon_stream.c b/src/remote/remote_daemon_stream.c
index 82cadb67ac..73e4d7befb 100644
--- a/src/remote/remote_daemon_stream.c
+++ b/src/remote/remote_daemon_stream.c
@@ -293,10 +293,25 @@ daemonStreamFilter(virNetServerClientPtr client,
     daemonClientStream *stream = opaque;
     int ret = 0;
 
+    /* We must honour lock ordering here. Client private data lock must
+     * be acquired before client lock. Bu we are already called with
+     * client locked. To avoid stream disappearing while we unlock
+     * everything, let's increase its refcounter. This has some
+     * implications though. */
+    stream->refs++;
     virObjectUnlock(client);
     virMutexLock(&stream->priv->lock);
     virObjectLock(client);
 
+    if (stream->refs == 1) {
+        /* So we are the only ones holding the reference to the stream.
+         * Return 1 to signal to the caller that we've processed the
+         * message. And to "process" means free. */
+        virNetMessageFree(msg);
+        ret = 1;
+        goto cleanup;
+    }
+
     if (msg->header.type != VIR_NET_STREAM &&
         msg->header.type != VIR_NET_STREAM_HOLE)
         goto cleanup;
@@ -318,6 +333,10 @@ daemonStreamFilter(virNetServerClientPtr client,
 
  cleanup:
     virMutexUnlock(&stream->priv->lock);
+    /* Don't pass client here, because client is locked here and this
+     * function might try to lock it again which would result in a
+     * deadlock. */
+    daemonFreeClientStream(NULL, stream);
     return ret;
 }
 
-- 
2.23.0