On 05.06.2019 12:09, Michal Privoznik wrote:
Changing domain definition must be guarded with acquiring modify job. The problem is if there is a thread executing say qemuDomainSetMemoryStatsPeriod() which is an API that acquires modify job and then possibly unlock the domain object and locks it again. However, becasue virDomainObjAssignDef() does not take a job (only object lock) it may have changed the domain definition while the other thread unlocked the domain object in order to talk on the monitor. For instance:
Thread1: 1) lookup domain object and lock it 2) acquire job 3) get pointers to live and persistent defs 4) unlock the domain object 5) talk to qemu on the monitor
Thread2 (Execute qemuDomainDefineXMLFlags): 1) lookup domain object and lock it 2) virDomainObjAssignDef() which overwrites persistent def and frees the old one 3) unlock domain object
Thread1: 6) lock the domain object again 7) try to access the persistent def via pointer stored in 3)
Now, this will crash because the pointer from step 3) points to a memory that was freed.
However, if Thread2 waited and acquired modify job (just like Thread1) then these two would serialize and no harm would be done.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com> --- src/qemu/qemu_domain.c | 55 +++++++++++++++++++++++++++++++++++++++ src/qemu/qemu_domain.h | 6 +++++ src/qemu/qemu_driver.c | 27 ++++++------------- src/qemu/qemu_migration.c | 5 +--- 4 files changed, 70 insertions(+), 23 deletions(-)
diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c index 4d3a8868b2..f6b677c69e 100644 --- a/src/qemu/qemu_domain.c +++ b/src/qemu/qemu_domain.c @@ -7055,6 +7055,61 @@ virDomainDefParserConfig virQEMUDriverDomainDefParserConfig = { };
+/** + * qemuDomainObjListAdd: + * @driver: qemu driver + * @def: domain definition + * @oldDef: previous domain definition + * @live: whether @def is live definition + * @flags: an bitwise-OR of virDomainObjListAdd flags + * + * Add a domain onto the list of domain object and sets its + * definition. If @oldDef is not NULL and there was pre-existing + * definition it's returned in @oldDef. + * + * In addition to that, if definition of an existing domain is + * changed a MODIFY job is acquired prior to that. + * + * Returns: domain object pointer on success, + * NULL otherwise. + */ +virDomainObjPtr +qemuDomainObjListAdd(virQEMUDriverPtr driver, + virDomainDefPtr def, + virDomainDefPtr *oldDef, + bool live, + unsigned int flags) +{ + virDomainObjPtr vm = NULL; + + if (!(vm = virDomainObjListAdd(driver->domains, def, driver->xmlopt, flags))) + return NULL; + + /* At this point, @vm is locked. If it doesn't have any + * definition set, then the call above just added it and + * there can't be anybody else using the object. It's safe to + * just set the definition without acquiring job. */ + if (!vm->def) { + virDomainObjAssignDef(vm, def, live, oldDef); + VIR_RETURN_PTR(vm); + } + + /* Bad luck. Domain was pre-existing and this call is trying + * to update its definition. Modify job MUST be acquired. */ + if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_MODIFY) < 0) { + qemuDomainRemoveInactive(driver, vm);
Here we can remove transient domain. Nikolay