-  core file analysis;

  - null pointer;

(gdb) frame 0
#0  virJSONValueObjectHasKey (object=object@entry=0x0, key=key@entry=0x7ff4e1ae6826 "error") at util/virjson.c:751
751	    if (object->type != VIR_JSON_TYPE_OBJECT)
(gdb) l
746	virJSONValueObjectHasKey(virJSONValuePtr object,
747	                         const char *key)
748	{
749	    size_t i;
750	
751	    if (object->type != VIR_JSON_TYPE_OBJECT)
752	        return -1;
753	
754	    for (i = 0; i < object->data.object.npairs; i++) {
755	        if (STREQ(object->data.object.pairs[i].key, key))

(gdb) p object
$12 = (virJSONValue *) 0x0


   - called by qemuAgentCheckError();

(gdb) frame 1
#1  0x00007ff4e19ee19d in qemuAgentCheckError (cmd=cmd@entry=0x7ff4cc06bbe0, reply=0x0) at qemu/qemu_agent.c:1098
1098	    if (virJSONValueObjectHasKey(reply, "error")) {
(gdb) l
1093	
1094	static int
1095	qemuAgentCheckError(virJSONValuePtr cmd,
1096	                    virJSONValuePtr reply)
1097	{
1098	    if (virJSONValueObjectHasKey(reply, "error")) {
1099	        virJSONValuePtr error = virJSONValueObjectGet(reply, "error");
1100	        char *cmdstr = virJSONValueToString(cmd, false);
1101	        char *replystr = virJSONValueToString(reply, false);



   - called by qemuAgentGetInterfaces();

(gdb) frame 2
#2  0x00007ff4e19f2040 in qemuAgentGetInterfaces (mon=mon@entry=0x7ff4a8001090, ifaces=ifaces@entry=0x7ff4ee95cad8) at qemu/qemu_agent.c:2070
2070	        qemuAgentCheckError(cmd, reply) < 0) {
(gdb) l
2065	
2066	    if (!(cmd = qemuAgentMakeCommand("guest-network-get-interfaces", NULL)))
2067	        goto cleanup;
2068	
2069	    if (qemuAgentCommand(mon, cmd, &reply, false, VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0 ||
2070	        qemuAgentCheckError(cmd, reply) < 0) {
2071	        goto cleanup;
2072	    }
2073	
2074	    if (!(ret_array = virJSONValueObjectGet(reply, "return"))) {


   - mon->running = false;

(gdb) p (* mon)->running
$8 = false


   - mon->await_event;

(gdb) p mon->await_event
$14 = QEMU_AGENT_EVENT_SHUTDOWN



   - VM "internet_021";

(gdb) frame 3
#3  0x00007ff4e1aac785 in qemuDomainInterfaceAddresses (dom=0x7ff4cc04a030, ifaces=0x7ff4ee95cad8, source=<optimized out>, flags=<optimized out>) at qemu/qemu_driver.c:20403
20403	        ret = qemuAgentGetInterfaces(agent, ifaces);
(gdb) p * dom
$13 = {object = {u = {dummy_align1 = 7700611123, dummy_align2 = 0x1cafe0033, s = {magic = 3405643827, refs = 1}}, klass = 0x7ff4d8152f60}, conn = 0x7ff4d811ab00, name = 0x7ff4cc06bc00 "internet_021", id = 312, 
  uuid = "\024v\214\266\300\235F\004\256\345\001\003\371L", <incomplete sequence \351>}



-  In qemuAgentCommand(), mon->running must have been 'true';

qemuAgentCommand(qemuAgentPtr mon,
                 virJSONValuePtr cmd,
                 virJSONValuePtr *reply,
                 bool needReply,
                 int seconds)
{
    int ret = -1;
    qemuAgentMessage msg;
    char *cmdstr = NULL;
    int await_event = mon->await_event;

    *reply = NULL;

    if (!mon->running) {
        virReportError(VIR_ERR_AGENT_UNRESPONSIVE, "%s",
                       _("Guest agent disappeared while executing command"));
        return -1;
    }

    if (qemuAgentGuestSync(mon) < 0)
        return -1;

    memset(&msg, 0, sizeof(msg));

    if (!(cmdstr = virJSONValueToString(cmd, false)))
        goto cleanup;
    if (virAsprintf(&msg.txBuffer, "%s" LINE_ENDING, cmdstr) < 0)
        goto cleanup;
    msg.txLength = strlen(msg.txBuffer);

    VIR_DEBUG("Send command '%s' for write, seconds = %d", cmdstr, seconds);

    ret = qemuAgentSend(mon, &msg, seconds);

    VIR_DEBUG("Receive command reply ret=%d rxObject=%p",
              ret, msg.rxObject);

    if (ret == 0) {
        /* If we haven't obtained any reply but we wait for an
         * event, then don't report this as error */
        if (!msg.rxObject) {
            if (await_event && !needReply) {
                VIR_DEBUG("Woken up by event %d", await_event);
            } else {
                if (mon->running)
                    virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                                   _("Missing monitor reply object"));
                else
                    virReportError(VIR_ERR_AGENT_UNRESPONSIVE, "%s",
                                   _("Guest agent disappeared while executing command"));
                ret = -1;
            }
        } else {
            *reply = msg.rxObject;
            ret = qemuAgentCheckError(cmd, *reply);
        }
    }



-  vdsm logs, VM's uuid = '14768cb6-c09d-4604-aee5-0103f94c79e9';

2018-12-20 04:05:52,898+0900 WARN  (qgapoller/56) [virt.periodic.VmDispatcher] could not run <function <lambda> at 0x7f6ba01d5aa0> on ['14768cb6-c09d-4604-aee5-0103f94c79e9', 'bccc97c6-8b6
2018-12-20 04:05:53,680+0900 INFO  (jsonrpc/7) [api.host] FINISH getAllVmIoTunePolicies return={'status': {'message': 'Done', 'code': 0}, 'io_tune_policies_dict': {'14768cb6-c09d-4604-aee5
2018-12-20 04:05:54,718+0900 WARN  (qgapoller/55) [virt.periodic.VmDispatcher] could not run <function <lambda> at 0x7f6ba01d5c80> on ['14768cb6-c09d-4604-aee5-0103f94c79e9', 'bccc97c6-8b6
2018-12-20 04:05:55,779+0900 WARN  (periodic/1251) [virt.periodic.VmDispatcher] could not run <class 'vdsm.virt.periodic.UpdateVolumes'> on ['14768cb6-c09d-4604-aee5-0103f94c79e9', 'bccc97
2018-12-20 04:06:02,812+0900 INFO  (vdsm.Scheduler) [Executor] Worker discarded: <Worker name=qgapoller/54 running <Task discardable <NetworkInterfacesCheck vm=14768cb6-c09d-4604-aee5-0103
2018-12-20 04:06:04,717+0900 WARN  (qgapoller/52) [virt.periodic.VmDispatcher] could not run <function <lambda> at 0x7f6ba01d5c80> on ['14768cb6-c09d-4604-aee5-0103f94c79e9', 'bccc97c6-8b6


-  sigsegv at 04:06:07;

$ grep -i sigsegv  ../messages 
Dec 20 04:06:07 host04 abrt-hook-ccpp: Process 7144 (libvirtd) of user 0 killed by SIGSEGV - dumping core



-  Assessment;

I think the guest agent must have been shutdown or terminated during the execution of this guest agent command. 

I have to admit, I have just realised that I made a mistake in my initial analysis. qemuAgentCheckError() is called from qemuAgentGetInterfaces() in tis case, but it is also called in qemuAgentCommand(). I had convinced myself that it was this latter call that was the failing one, and that had me confused. Then when I started to write this bug update, I saw/remembered that the "failing one" was actually called in qemuAgentGetInterfaces().

This was on a RHV host. The vdsm logs (above) show that the VM in question, 'internet_021', started to get qgapoller warnings at 04:05:52, then the sigsegv happened at 04:06:07. The qgapoller is a mechanism in VDSM that polls/queries the guest agent on a cyclic basis. So, it was having problems accessing the guest agent just prior to the core dump.

This occurred on a 7.5 version of libvirt, libvirt-3.9.0-14.el7_5.6. However, I checked the latest d/s version, libvirt-4.5.0-10.el7_6.3, and the code appears to be the same in these areas in both versions.

This was possibly fixed by:
https://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=9ed175fbc2deecfdaeabca7bc77c7e7ae33a3377
commit 9ed175fbc2deecfdaeabca7bc77c7e7ae33a3377
Author:     John Ferlan <jferlan>
CommitDate: 2018-09-11 10:32:01 -0400

    qemu: Remove duplicated qemuAgentCheckError
    
    Commit 5b3492fadb moved qemuAgentCheckError calls into
    qemuAgentCommand for various reasons; however, subsequent
    commit 0977b8aa0 adding a new command made call again
    So let's just remove the duplicitous call from
    qemuAgentGetInterfaces.
    
    Signed-off-by: John Ferlan <jferlan>
    ACKed-by: Michal Privoznik <mprivozn>

git describe: v4.7.0-83-g9ed175fbc2 contains: v4.8.0-rc1~226

I can reproduce this race condition issue by call domifaddr --agent during shutdown guest (like bug 1058149).

After some debuging, i think virJSONValueObjectGet(reply, "return") still will crash libvirtd after John's commit 9ed175fb,
since virJSONValueObjectGet won't check if @object is null and this will crash libvirtd. And I think maybe the right fix is
to set needReply to true during call qemuAgentCommand() in qemuAgentGetInterfaces(), then qemuAgentCommand won't return 0 with
a null @reply pointer, and also from the design this qemu agent command will return some guest info, it really "needReply" from qemu agent.

Obviously this is just my opinion, corrections are most welcome.

Right. That commit merely changed the point where libvirtd will crash.

I've sent the patch to libvirt-security, since this might possibly be used maliciously.

Here is my reproducing steps:

1. In guest (already install qemu-guest-agent):

# cat script.gdb 
set width 0
set height 0
set verbose off

b qmp_guest_network_get_interfaces
commands 1
  kill
end

b qmp_guest_shutdown
commands 2
  return
  c
end

c

# gdb -batch --command=script.gdb qemu-ga `pidof qemu-ga`

2. On host:

# virsh shutdown vm1 --mode agent; virsh domifaddr vm1 --source agent
error: Failed to shutdown domain vm1
error: Guest agent is not responding: Guest agent not available for now

error: Disconnected from qemu:///system due to end of file
error: Failed to query for interfaces addresses
error: End of file while reading data: Input/output error

Patch sent upstream:
https://www.redhat.com/archives/libvir-list/2019-January/msg00241.html

Pushed upstream as:
commit 7cfd1fbb1332ae5df678b9f41a62156cb2e88c73
Author:     Ján Tomko <jtomko>
CommitDate: 2019-01-10 12:09:48 +0100

    qemu: require reply from guest agent in qemuAgentGetInterfaces
    
git describe: v4.10.0-164-g7cfd1fbb13

Verify this bug with libvirt-4.5.0-18.el7.x86_64:

1. In guest (already install qemu-guest-agent):

# cat script.gdb 
set width 0
set height 0
set verbose off

b qmp_guest_network_get_interfaces
commands 1
  kill
end

b qmp_guest_shutdown
commands 2
  return
  c
end

c

# gdb -batch --command=script.gdb qemu-ga `pidof qemu-ga`

2. On host:
# virsh shutdown vm1 --mode agent; virsh domifaddr vm1 --source agent
error: Failed to shutdown domain vm1
error: Guest agent is not responding: Guest agent not available for now

error: Failed to query for interfaces addresses
error: Guest agent is not responding: Guest agent disappeared while executing command

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2019:2294