From b7c98329cb055422d6e70262820485fca2045d09 Mon Sep 17 00:00:00 2001 From: Peter Krempa Date: Tue, 9 Apr 2013 13:56:26 +0200 Subject: [PATCH] conf: Fix race between looking up a domain object and freeing it This patch fixes crash of the daemon that happens due to the following race condition: Let's have two threads in the libvirtd daemon's qemu driver: A - thread executing undefine on the same domain B - thread executing a API call to get information about a domain Assume following serialization of operations done by the threads: 1) A has the lock on the domain object and is executing some code prior to virDomainObjListRemove() 2) B takes the lock on the domain object list, looks up the domain object pointer and blocks in the attempt to lock the domain object as A is holding the lock 3) A reaches virDomainObjListRemove() and unlocks the lock on the domain object 4) A blocks on the attempt to get the domain list lock 5) B is able to lock the domain object now and unlocks the domain list 6) A is now able to lock the domain list, and sheds the last reference on the domain object, this triggers the freeing function. 6) B starts executing the code on the pointer that is being freed 7) The libvirtd daemon crashes while attempting to access invalid pointer in thread B. This patch fixes the race by acquiring a reference on the domain object before unlocking it in virDomainObjListRemove() and re-locks the object prior to removing and freeing it. This ensures that no thread holds a lock on the domain object at the time it is removed from the list, and that doing a list lookup will never find a domain that is about to vanish. This is a minimal fix of the problem, but a better solution will be to switch to full reference counting for domain objects. --- src/conf/domain_conf.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c index 492e0b73a4..e00a5321f9 100644 --- a/src/conf/domain_conf.c +++ b/src/conf/domain_conf.c @@ -2238,10 +2238,14 @@ void virDomainObjListRemove(virDomainObjListPtr doms, char uuidstr[VIR_UUID_STRING_BUFLEN]; virUUIDFormat(dom->def->uuid, uuidstr); + virObjectRef(dom); virObjectUnlock(dom); virObjectLock(doms); + virObjectLock(dom); virHashRemoveEntry(doms->objs, uuidstr); + virObjectUnlock(dom); + virObjectUnref(dom); virObjectUnlock(doms); }