[libvirt] [PATCH] events: Avoid double free possibility on remote call failure
Michal Privoznik
mprivozn at redhat.com
Sun Jun 25 08:51:08 UTC 2017
On 06/25/2017 12:34 AM, John Ferlan wrote:
>
>
> On 06/24/2017 01:13 PM, Michal Privoznik wrote:
>> 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 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.
>>
>> 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?
>>
>
> I'm chuckling ;-) usually it's the double negatives that get me, but
> because I was solving the problem of needing to not call the FreeCb, I
> went with the inhibit option.
>
> I can flip-flop though... Instead of inhibitFreeCb, I can change to
> doFreeCb and repost.
Okay, ACK if you fix that.
Michal
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