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(a)huawei.com&gt;. >>> >>> Signed-off-by: John Ferlan <jferlan(a)redhat.com&gt; >>> --- >>> 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?