From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Stefan Reiter <s.reiter@proxmox.com>
Date: Mon, 23 Aug 2021 11:28:32 +0200
Subject: [PATCH] monitor/qmp: fix race with clients disconnecting early

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 everytime 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), 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>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
---
 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 737e750670..38804b8595 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 a2cdbbf646..b531bd50e7 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -152,6 +152,13 @@ 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 86949024f6..c306cadcf4 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -135,6 +135,21 @@ 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 092c527b6f..6b8cfcf6d8 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 0990873ec8..e605003771 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -117,16 +117,28 @@ 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);
 }
 
@@ -231,6 +243,7 @@ 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 afa477aae6..d3ff124bf3 100644
--- a/stubs/monitor-core.c
+++ b/stubs/monitor-core.c
@@ -12,6 +12,11 @@ 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)
 {
 }
