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



On 06/24/2017 01:26 PM, John Ferlan wrote:
> 
> 
> 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 huawei com>.
>>>
>>> Signed-off-by: John Ferlan <jferlan 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.

Ah, the negated logic confused me. I think I'd be nicer if we've used
boolean in straight logic (= do call) instead of reversed (= don't call).

What do you think?

Michal


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