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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7b18d2aac97sm528617485a.62.2024.10.30.10.51.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Oct 2024 10:51:57 -0700 (PDT) Date: Wed, 30 Oct 2024 13:51:54 -0400 From: Peter Xu To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: qemu-devel@nongnu.org, Markus Armbruster , Peter Maydell , Eduardo Habkost , Paolo Bonzini , Mark Cave-Ayland , Igor Mammedov , "Michael S . Tsirkin" , Alex Williamson , "Dr . David Alan Gilbert" , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , =?utf-8?Q?C=C3=A9dric?= Le Goater , Fabiano Rosas , Juraj Marcin Subject: Re: [PATCH RFC v2 0/7] QOM: Singleton interface Message-ID: References: <20241029211607.2114845-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.366, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Oct 30, 2024 at 04:13:57PM +0000, Daniel P. Berrangé wrote: > On Wed, Oct 30, 2024 at 09:13:13AM -0400, Peter Xu wrote: > > On Wed, Oct 30, 2024 at 09:48:07AM +0000, Daniel P. Berrangé wrote: > > > On Tue, Oct 29, 2024 at 05:16:00PM -0400, Peter Xu wrote: > > > > v1: https://lore.kernel.org/r/20241024165627.1372621-1-peterx@redhat.com > > > > > > > Meanwhile, migration has a long standing issue on current_migration > > > > pointer, where it can point to freed data after the migration object is > > > > finalized. It is debatable that the pointer can be cleared after the main > > > > thread (1) join() the migration thread first, then (2) release the last > > > > refcount for the migration object and clear the pointer. However there's > > > > still major challenges [1]. With singleton, we could have a slightly but > > > > hopefully working workaround to clear the pointer during finalize(). > > > > > > I'm still not entirely convinced that this singleton proposal is > > > fixing the migration problem correctly. > > > > > > Based on discussions in v1, IIUC, the situation is that we have > > > migration_shutdown() being called from qemu_cleanup(). The former > > > will call object_unref(current_migration), but there may still > > > be background migration threads running that access 'current_migration', > > > and thus a potential use-after-free. > > > > migration thread is fine, it takes a refcount at the entry. > > > > And btw, taking it at the entry is racy, we've just fixed it, see (in my > > next migration pull): > > > > https://lore.kernel.org/qemu-devel/20241024213056.1395400-2-peterx@redhat.com/ > > Yep, acquiring the refcount immediately before thread-create > is what I meant. > > > The access reported was, IIUC, outside migration code, but after both > > main/migration threads released the refcount, hence after finalize(). It > > could be a random migration_is_running() call very late in device code, for > > example. > > > > > > > > > > > Based on what the 7th patch here does, the key difference is that > > > the finalize() method for MigrationState will set 'current_migration' > > > to NULL after free'ing it. > > > > Yes. But this show case series isn't complete. We need a migration-side > > lock finally to make it safe to access. For that, see: > > > > https://lore.kernel.org/qemu-devel/20241024213056.1395400-9-peterx@redhat.com/ > > > > > > > > I don't believe that is safe. > > > > I hope after the other series applied it will be 100% safe, even though I > > agree it's tricky. But hopefully QOM is very clean, the trickly part is > > still within migration, and it should be less tricky than migration > > implement a refcount on top of Object.. > > Ok, so with the other series applied, this does look safe, but > it also doesn't seem to really have any dependancy on the > single interface code. Patch 7 here looks sufficient, in combo > with the other 2 series to avoid the use-after-free flaws. Patch 7, when applied without patch 6 and prior, will crash in device-introspect-test, trying to create yet another migration object when processing the "device-list-properties" QMP command. And it turns out that's also not the only way QEMU can crash by that. Fundamentally it's because patch 7 has global operations within init()/finalize() to fix the migration dangling pointer, hence it must not be instanciated more than once. It's also probably because I always think singleton can be useful in general to QEMU's device model where can be special devices all over the places that I'm not aware of. I didn't work on a lot of QEMU devices, but with that limited experience I still stumbled upon two devices (if taking migration object as one..) that might benefit from it. That leads to this whole series, which is also the cleanest so far I can think of to solve the immediate migration UAF. Thanks, > > > I think the question is whether things like migration_is_running() is > > allowed to be used anywhere, even after migration_shutdown(). My answer > > is, it should be ok to be used anywhere, and we don't necessarilly need to > > limit that. In that case the caller doesn't need to take a refcount > > because it's an immediate query. It can simply check its existance with > > the lock (after my patch 8 of the other series applied, which depends on > > this qom series). > > Agree, and from a practical POV, I think it would be impossible to > require a ref count be held from other non-migration threads, so the > locking around 'current_migration' looks like the only practical > option. > > With regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| > -- Peter Xu