[libvirt] [PATCH] events: Avoid double free possibility on remote call failure

[John Ferlan]

On 06/23/2017 05:39 AM, Michal Privoznik wrote:
> On 06/23/2017 12:10 AM, John Ferlan wrote:
>> If a remote call fails during event registration (more than likely from
>> a network failure or remote libvirtd restart timed just right), then when
>> calling the virObjectEventStateDeregisterID we don't want to call the
>> registered @freecb function because that breaks our contract that we
>> would only call it after succesfully returning.  If the @freecb routine
>> were called, it could result in a double free from properly coded
>> applications that free their opaque data on failure to register, as seen
>> in the following details:
>>
>>     Program terminated with signal 6, Aborted.
>>     #0  0x00007fc45cba15d7 in raise
>>     #1  0x00007fc45cba2cc8 in abort
>>     #2  0x00007fc45cbe12f7 in __libc_message
>>     #3  0x00007fc45cbe86d3 in _int_free
>>     #4  0x00007fc45d8d292c in PyDict_Fini
>>     #5  0x00007fc45d94f46a in Py_Finalize
>>     #6  0x00007fc45d960735 in Py_Main
>>     #7  0x00007fc45cb8daf5 in __libc_start_main
>>     #8  0x0000000000400721 in _start
>>
>> The double dereference of 'pyobj_cbData' is triggered in the following way:
>>
>>     (1) libvirt_virConnectDomainEventRegisterAny is invoked.
>>     (2) the event is successfully added to the event callback list
>>         (virDomainEventStateRegisterClient in
>>         remoteConnectDomainEventRegisterAny returns 1 which means ok).
>>     (3) when function remoteConnectDomainEventRegisterAny is hit,
>>         network connection disconnected coincidently (or libvirtd is
>>         restarted) in the context of function 'call' then the connection
>>         is lost and the function 'call' failed, the branch
>>         virObjectEventStateDeregisterID is therefore taken.
>>     (4) 'pyobj_conn' is dereferenced the 1st time in
>>         libvirt_virConnectDomainEventFreeFunc.
>>     (5) 'pyobj_cbData' (refered to pyobj_conn) is dereferenced the
>>          2nd time in libvirt_virConnectDomainEventRegisterAny.
>>     (6) the double free error is triggered.
>>
>> Resolve this by adding an @inhibitFreeCb boolean in order to avoid calling
>> freecb in virObjectEventStateDeregisterID for any remote call failure in
>> a remoteConnect*EventRegister* API. For remoteConnect*EventDeregister* calls,
>> the passed value would be false indicating they should run the freecb if it
>> exists.
>>
>> Patch based on the investigation and initial patch posted by:
>> fangying <fangying1 at huawei.com>.
>>
>> Signed-off-by: John Ferlan <jferlan at redhat.com>
>> ---
>>   Initial patch found at:
>>
>>   https://www.redhat.com/archives/libvir-list/2017-June/msg00039.html
>>
>>   based on feedback from Daniel Berrange to a previous posting:
>>
>>   https://www.redhat.com/archives/libvir-list/2017-May/msg01089.html
>>
>>   Since no new patch was posted, I posted my idea from review for
>>   consideration.
>>
>>  src/bhyve/bhyve_driver.c             |  2 +-
>>  src/conf/domain_event.c              |  2 +-
>>  src/conf/object_event.c              | 23 +++++++++++++++++------
>>  src/conf/object_event.h              |  3 ++-
>>  src/libxl/libxl_driver.c             |  2 +-
>>  src/lxc/lxc_driver.c                 |  2 +-
>>  src/network/bridge_driver.c          |  2 +-
>>  src/node_device/node_device_driver.c |  2 +-
>>  src/qemu/qemu_driver.c               |  4 ++--
>>  src/remote/remote_driver.c           | 32 ++++++++++++++++----------------
>>  src/secret/secret_driver.c           |  2 +-
>>  src/storage/storage_driver.c         |  2 +-
>>  src/test/test_driver.c               |  8 ++++----
>>  src/uml/uml_driver.c                 |  2 +-
>>  src/vz/vz_driver.c                   |  2 +-
>>  src/xen/xen_driver.c                 |  2 +-
>>  16 files changed, 52 insertions(+), 40 deletions(-)
>>
>> diff --git a/src/bhyve/bhyve_driver.c b/src/bhyve/bhyve_driver.c
>> index ed2221a..6722dc4 100644
>> --- a/src/bhyve/bhyve_driver.c
>> +++ b/src/bhyve/bhyve_driver.c
>> @@ -1503,7 +1503,7 @@ bhyveConnectDomainEventDeregisterAny(virConnectPtr conn,
>>  
>>      if (virObjectEventStateDeregisterID(conn,
>>                                          privconn->domainEventState,
>> -                                        callbackID) < 0)
>> +                                        callbackID, false) < 0)
>>          return -1;
>>  
>>      return 0;
>> diff --git a/src/conf/domain_event.c b/src/conf/domain_event.c
>> index 6e471d7..ff4c602 100644
>> --- a/src/conf/domain_event.c
>> +++ b/src/conf/domain_event.c
>> @@ -2301,7 +2301,7 @@ virDomainEventStateDeregister(virConnectPtr conn,
>>                                                 NULL);
>>      if (callbackID < 0)
>>          return -1;
>> -    return virObjectEventStateDeregisterID(conn, state, callbackID);
>> +    return virObjectEventStateDeregisterID(conn, state, callbackID, false);
> 
> Why is this false? virDomainEventStateDeregister is called directly from
> public API implementations. For instance from
> qemuConnectDomainEventDeregister(). Don't we want to run free callback then?
> 

and the eventual check in virObjectEventCallbackListRemoveID is:

        /* inhibiting calling @freecb from error paths in register
         * functions ensures the caller of the register function won't
         * end up with a double free error */
        if (!inhibitFreeCb && cb->freecb)

IOW: Any time called from a Deregister path, we call the free callback;
however, when called from a RPC call REGISTER path, then we don't want
to call the free callback because we're returning -1 to the caller and
the caller will free things.

Or am I misreading your question. For a Deregister path if the
cb->freecb exists, then we want to call it like we have.

John


>>  }
>>  
> 
> Michal
>