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

[Michal Privoznik]

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?

>  }
>  

Michal