[PATCH] monitor/qmp: fix race with clients disconnecting early

[Stefan Reiter]

From: Stefan Reiter <stefan@pimaker.at>

The following sequence can produce a race condition that results in
responses meant for different clients being sent to the wrong one:

(QMP, no OOB)
1) client A connects
2) client A sends 'qmp_capabilities'
3) 'qmp_dispatch' runs in coroutine, schedules out to
   'do_qmp_dispatch_bh' and yields
4) client A disconnects (i.e. aborts, crashes, etc...)
5) client B connects
6) 'do_qmp_dispatch_bh' runs 'qmp_capabilities' and wakes calling coroutine
7) capabilities are now set and 'mon->commands' is set to '&qmp_commands'
8) 'qmp_dispatch' returns to 'monitor_qmp_dispatch'
9) success message is sent to client B *without it ever having sent
   'qmp_capabilities' itself*
9a) even if client B ignores it, it will now presumably send it's own
   greeting, which will error because caps are already set

The fix proposed here uses an atomic, sequential connection number
stored in the MonitorQMP struct, which is incremented every time a new
client connects. Since it is not changed on CHR_EVENT_CLOSED, the
behaviour of allowing a client to disconnect only one side of the
connection is retained.

The connection_nr needs to be exposed outside of the monitor subsystem,
since qmp_dispatch lives in qapi code. It needs to be checked twice,
once for actually running the command in the BH (fixes 7/9a), and once for
sending back a response (fixes 9).

This satisfies my local reproducer - using multiple clients constantly
looping to open a connection, send the greeting, then exiting no longer
crashes other, normally behaving clients with unrelated responses.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---

This is another patch in the apparently never-ending series of fixes to
QMP-related race conditions. Our PVE users really seem to have a knack for
triggering these ;)

Here's one of the (several) forum threads where we tried to diagnose the issue:
https://forum.proxmox.com/threads/error-with-backup-when-backing-up-qmp-command-query-backup-failed-got-wrong-command-id.88017/

It manifested itself under load, where sometimes our monitor code (which uses
the 'id' system of QMP to track commands) would receive bogus responses, showing
up as "got wrong command id" errors in our logs.

I'm not sure this approach is the best fix, it seems a lot like a band-aid to
me, but it's the best I could come up with for now - open for different ideas of
course.

Note that with this patch, we're no longer running a command that was scheduled
after a client has disconnected early. This seems like a slight behaviour change
to me... On the other hand, I didn't want to drag the connection number into
qmp_capabilities() and special case that even further.

I didn't look to deeply into 'QMP in iothread' yet, if that does what I think it
does there might be two more potential races here:
* between 'do_qmp_dispatch_bh' running 'aio_co_wake' and 'qmp_dispatch' actually
  yielding (very unlikely though)
* plus a TOCTOU in 'do_qmp_dispatch_bh' with the atomic read of the
  connection_nr and actually running cmd->fn()

Thanks!


 include/monitor/monitor.h  |  1 +
 monitor/monitor-internal.h |  7 +++++++
 monitor/monitor.c          | 15 +++++++++++++++
 monitor/qmp.c              | 15 ++++++++++++++-
 qapi/qmp-dispatch.c        | 21 +++++++++++++++++----
 stubs/monitor-core.c       |  5 +++++
 6 files changed, 59 insertions(+), 5 deletions(-)

diff --git a/include/monitor/monitor.h b/include/monitor/monitor.h
index af3887bb71..ff6db1448b 100644
--- a/include/monitor/monitor.h
+++ b/include/monitor/monitor.h
@@ -16,6 +16,7 @@ extern QemuOptsList qemu_mon_opts;
 Monitor *monitor_cur(void);
 Monitor *monitor_set_cur(Coroutine *co, Monitor *mon);
 bool monitor_cur_is_qmp(void);
+int monitor_get_connection_nr(const Monitor *mon);
 
 void monitor_init_globals(void);
 void monitor_init_globals_core(void);
diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index 9c3a09cb01..a92be8c3f7 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -144,6 +144,13 @@ typedef struct {
     QemuMutex qmp_queue_lock;
     /* Input queue that holds all the parsed QMP requests */
     GQueue *qmp_requests;
+
+    /*
+     * A sequential number that gets incremented on every new CHR_EVENT_OPENED.
+     * Used to avoid leftover responses in BHs from being sent to the wrong
+     * client. Access with atomics.
+     */
+    int connection_nr;
 } MonitorQMP;
 
 /**
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 636bcc81c5..ee5ac26c37 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -136,6 +136,21 @@ bool monitor_cur_is_qmp(void)
     return cur_mon && monitor_is_qmp(cur_mon);
 }
 
+/**
+ * If @mon is a QMP monitor, return the connection_nr, otherwise -1.
+ */
+int monitor_get_connection_nr(const Monitor *mon)
+{
+    MonitorQMP *qmp_mon;
+
+    if (!monitor_is_qmp(mon)) {
+        return -1;
+    }
+
+    qmp_mon = container_of(mon, MonitorQMP, common);
+    return qatomic_read(&qmp_mon->connection_nr);
+}
+
 /**
  * Is @mon is using readline?
  * Note: not all HMP monitors use readline, e.g., gdbserver has a
diff --git a/monitor/qmp.c b/monitor/qmp.c
index 0ef7cebb78..3ec67e32d3 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -141,6 +141,8 @@ static void monitor_qmp_dispatch(MonitorQMP *mon, QObject 
*req)
     QDict *rsp;
     QDict *error;
 
+    int conn_nr_before = qatomic_read(&mon->connection_nr);
+
     rsp = qmp_dispatch(mon->commands, req, qmp_oob_enabled(mon),
                        &mon->common);
 
@@ -156,7 +158,17 @@ static void monitor_qmp_dispatch(MonitorQMP *mon, QObject 
*req)
         }
     }
 
-    monitor_qmp_respond(mon, rsp);
+    /*
+     * qmp_dispatch might have yielded and waited for a BH, in which case there
+     * is a chance a new client connected in the meantime - if this happened,
+     * the command will not have been executed, but we also need to ensure that
+     * we don't send back a corresponding response on a line that no longer
+     * belongs to this request.
+     */
+    if (conn_nr_before == qatomic_read(&mon->connection_nr)) {
+        monitor_qmp_respond(mon, rsp);
+    }
+
     qobject_unref(rsp);
 }
 
@@ -444,6 +456,7 @@ static void monitor_qmp_event(void *opaque, QEMUChrEvent 
event)
 
     switch (event) {
     case CHR_EVENT_OPENED:
+        qatomic_inc_fetch(&mon->connection_nr);
         mon->commands = &qmp_cap_negotiation_commands;
         monitor_qmp_caps_reset(mon);
         data = qmp_greeting(mon);
diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
index 59600210ce..95602446eb 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -120,16 +120,28 @@ typedef struct QmpDispatchBH {
     QObject **ret;
     Error **errp;
     Coroutine *co;
+    int conn_nr;
 } QmpDispatchBH;
 
 static void do_qmp_dispatch_bh(void *opaque)
 {
     QmpDispatchBH *data = opaque;
 
-    assert(monitor_cur() == NULL);
-    monitor_set_cur(qemu_coroutine_self(), data->cur_mon);
-    data->cmd->fn(data->args, data->ret, data->errp);
-    monitor_set_cur(qemu_coroutine_self(), NULL);
+    /*
+     * A QMP monitor tracks it's client with a connection number, if this
+     * changes during the scheduling delay of this BH, we must not execute the
+     * command. Otherwise a badly placed 'qmp_capabilities' might affect the
+     * connection state of a client it was never meant for.
+     */
+    if (data->conn_nr == monitor_get_connection_nr(data->cur_mon)) {
+        assert(monitor_cur() == NULL);
+        monitor_set_cur(qemu_coroutine_self(), data->cur_mon);
+        data->cmd->fn(data->args, data->ret, data->errp);
+        monitor_set_cur(qemu_coroutine_self(), NULL);
+    } else {
+        error_setg(data->errp, "active monitor connection changed");
+    }
+
     aio_co_wake(data->co);
 }
 
@@ -243,6 +255,7 @@ QDict *qmp_dispatch(const QmpCommandList *cmds, QObject 
*request,
             .ret        = &ret,
             .errp       = &err,
             .co         = qemu_coroutine_self(),
+            .conn_nr    = monitor_get_connection_nr(cur_mon),
         };
         aio_bh_schedule_oneshot(qemu_get_aio_context(), do_qmp_dispatch_bh,
                                 &data);
diff --git a/stubs/monitor-core.c b/stubs/monitor-core.c
index d058a2a00d..3290b58120 100644
--- a/stubs/monitor-core.c
+++ b/stubs/monitor-core.c
@@ -13,6 +13,11 @@ Monitor *monitor_set_cur(Coroutine *co, Monitor *mon)
     return NULL;
 }
 
+int monitor_get_connection_nr(const Monitor *mon)
+{
+    return -1;
+}
+
 void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
 {
 }
-- 
2.30.2