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bh=Rt/QP2soUo8lehryJeNw+e8WlCIWnqojq+ZE/t/0ujI=; b=EaFGD8zqbsFh2hnEPuZuDlZtmI4syHi5lIVV64p66yPuQZ/BNuv2HukjI00C4Hnb9Gkp6H FTKPrsAMdBBVozAQ== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 332FF13A42; Thu, 20 Feb 2025 15:56:34 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id XVg9OLJQt2dSeQAAD6G6ig (envelope-from ); Thu, 20 Feb 2025 15:56:34 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Juraj Marcin , Yan Fu Subject: Re: [PATCH] migration: Fix UAF for incoming migration on MigrationState In-Reply-To: References: <20250220132459.512610-1-peterx@redhat.com> <87h64o90dn.fsf@suse.de> Date: Thu, 20 Feb 2025 12:56:32 -0300 Message-ID: <87cyfc8v9r.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Queue-Id: C0ECD1F387 X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; ARC_NA(0.00)[]; ASN(0.00)[asn:25478, ipnet:::/0, country:RU]; MISSING_XM_UA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; TO_DN_SOME(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; RCPT_COUNT_THREE(0.00)[4]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:email,suse.de:dkim,suse.de:mid]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:2; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, 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 Peter Xu writes: > On Thu, Feb 20, 2025 at 11:06:12AM -0300, Fabiano Rosas wrote: >> Peter Xu writes: >> >> > On the incoming migration side, QEMU uses a coroutine to load all the VM >> > states. Inside, it may reference MigrationState on global states like >> > migration capabilities, parameters, error state, shared mutexes and more. >> > >> > However there's nothing yet to make sure MigrationState won't get >> > destroyed (e.g. after migration_shutdown()). Meanwhile there's also no API >> > available to remove the incoming coroutine in migration_shutdown(), >> > avoiding it to access the freed elements. >> > >> > There's a bug report showing this can happen and crash dest QEMU when >> > migration is cancelled on source. >> > >> > When it happens, the dest main thread is trying to cleanup everything: >> > >> > #0 qemu_aio_coroutine_enter >> > #1 aio_dispatch_handler >> > #2 aio_poll >> > #3 monitor_cleanup >> > #4 qemu_cleanup >> > #5 qemu_default_main >> > >> > Then it found the migration incoming coroutine, schedule it (even after >> > migration_shutdown()), causing crash: >> > >> > #0 __pthread_kill_implementation >> > #1 __pthread_kill_internal >> > #2 __GI_raise >> > #3 __GI_abort >> > #4 __assert_fail_base >> > #5 __assert_fail >> > #6 qemu_mutex_lock_impl >> > #7 qemu_lockable_mutex_lock >> > #8 qemu_lockable_lock >> > #9 qemu_lockable_auto_lock >> > #10 migrate_set_error >> > #11 process_incoming_migration_co >> > #12 coroutine_trampoline >> > >> > To fix it, take a refcount after an incoming setup is properly done when >> > qmp_migrate_incoming() succeeded the 1st time. As it's during a QMP >> > handler which needs BQL, it means the main loop is still alive (without >> > going into cleanups, which also needs BQL). >> >> We should start documenting uses of BQL and dependencies on the main >> loop more thoroughly. Otherwise later when we decide to move stuff into >> threads or QMP people decide to rework how QMP uses coroutines, >> etc. there we'll be many bugs. > > Yeh better documentation is always good. Maybe there're too many things > depend on BQL so it's not easy to provide a document. Normally, afaiu, we > document the other way round, where things do not need BQL. > Well, for newly added code the developer should know about the data structures they're using and what the dependencies are. But yeah, there's subtleties and hidden assumptions everywhere. >> >> I think the BQL is irrelevant here. The concurrent access is prevented >> by qmp_migrate_incoming() not being a coroutine, hence keeping the main >> loop from looping. >> >> This case would be "relying on the qmp_migrate_incoming() being >> serialized with the dispatch of the incoming coroutine by the main >> loop". > > I checked just now, it's still true indeed now that both of them > (qmp_migrate_incoming, and the cleanup code) need to be run in the main > thread. But I'll not be surprised if someone moves (or moved) it out into > a separate iothread like what we do with the OOB commands. > Yes, that's my point on documentation. "Needs BQL because I said so" doesn't help when doing that kind of work. AFAIK we're suck in this QMP-needs-BQL mode precisely because we cannot determine clearly what the dependencies are. There's been some work years ago to move QMP commands into coroutines. Now we have a half-baked situation. Grep for QCO_COROUTINE. > When I was working on OOB stuff, I _think_ all things are ready to create > yet another iothread to process cmds need bql, probably just not necessary. > Fundamentally, the requirement, AFAIU, is qmp handlers run by default with > BQL, and it doesn't need to always be on the main thread. > > Let's keep the "BQL" term? I think I'm ok with your version as it states > the facts at least as of now, but I still think BQL is the slightly more > accurate term. > Fine. >> >> > >> > Releasing the refcount now only until the incoming migration coroutine >> > finished or failed. Hence the refcount is valid for both (1) setup phase >> > of incoming ports, mostly IO watches (e.g. qio_channel_add_watch_full()), >> > and (2) the incoming coroutine itself (process_incoming_migration_co()). >> > >> > Note that we can't unref in migration_incoming_state_destroy(), because >> > both qmp_xen_load_devices_state() and load_snapshot() will use it without >> > an incoming migration. Those hold BQL so they're not prone to this issue. >> > >> > PS: I suspect nobody uses Xen's command at all, as it didn't register yank, >> > hence AFAIU the command should crash on master when trying to unregister >> > yank in migration_incoming_state_destroy().. but that's another story. >> > >> > Also note that in some incoming failure cases we may not always unref the >> > MigrationState refcount, which is a trade-off to keep things simple. We >> > could make it accurate, but it can be an overkill. Some examples: >> > >> > - Unlike most of the rest protocols, socket_start_incoming_migration() >> > may create net listener after incoming port setup sucessfully. >> > It means we can't unref in migration_channel_process_incoming() as a >> > generic path because socket protocol might keep using MigrationState. >> > >> > - For either socket or file, multiple IO watches might be created, it >> > means logically each IO watch needs to take one refcount for >> > MigrationState so as to be 100% accurate on ownership of refcount taken. >> > >> > In general, we at least need per-protocol handling to make it accurate, >> > which can be an overkill if we know incoming failed after all. Add a short >> > comment to explain that when taking the refcount in qmp_migrate_incoming(). >> > >> > Bugzilla: https://issues.redhat.com/browse/RHEL-69775 >> > Tested-by: Yan Fu >> > Signed-off-by: Peter Xu >> > --- >> > migration/migration.c | 40 ++++++++++++++++++++++++++++++++++++++-- >> > 1 file changed, 38 insertions(+), 2 deletions(-) >> > >> > diff --git a/migration/migration.c b/migration/migration.c >> > index c597aa707e..f57d853e9f 100644 >> > --- a/migration/migration.c >> > +++ b/migration/migration.c >> > @@ -116,6 +116,27 @@ static void migration_downtime_start(MigrationState *s) >> > s->downtime_start = qemu_clock_get_ms(QEMU_CLOCK_REALTIME); >> > } >> > >> > +/* >> > + * This is unfortunate: incoming migration actually needs the outgoing >> > + * migration state (MigrationState) to be there too, e.g. to query >> > + * capabilities, parameters, using locks, setup errors, etc. >> > + * >> > + * NOTE: when calling this, making sure current_migration exists and not >> > + * been freed yet! Otherwise trying to access the refcount is already >> > + * an use-after-free itself.. >> > + * >> > + * TODO: Move shared part of incoming / outgoing out into separate object. >> > + * Then this is not needed. >> >> It will be needed on the new object still, no? > > In that case IIUC we don't need special treatment for incoming side like > this, but only when QEMU starts it grabs that common object ref once, > either release it at the very end of qemu, or just make it static. > >> >> > + */ >> > +static void migrate_incoming_ref_outgoing_state(void) >> > +{ >> > + object_ref(migrate_get_current()); >> > +} >> > +static void migrate_incoming_unref_outgoing_state(void) >> > +{ >> > + object_unref(migrate_get_current()); >> > +} >> > + >> > static void migration_downtime_end(MigrationState *s) >> > { >> > int64_t now = qemu_clock_get_ms(QEMU_CLOCK_REALTIME); >> > @@ -850,7 +871,7 @@ process_incoming_migration_co(void *opaque) >> > * postcopy thread. >> > */ >> > trace_process_incoming_migration_co_postcopy_end_main(); >> > - return; >> > + goto out; >> > } >> > /* Else if something went wrong then just fall out of the normal exit */ >> > } >> > @@ -866,7 +887,8 @@ process_incoming_migration_co(void *opaque) >> > } >> > >> > migration_bh_schedule(process_incoming_migration_bh, mis); >> > - return; >> > + goto out; >> > + >> > fail: >> > migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE, >> > MIGRATION_STATUS_FAILED); >> > @@ -883,6 +905,9 @@ fail: >> > >> > exit(EXIT_FAILURE); >> > } >> > +out: >> > + /* Pairs with the refcount taken in qmp_migrate_incoming() */ >> > + migrate_incoming_unref_outgoing_state(); >> >> Nit, the comment is redundant, the function name is already clear >> enough. > > The function says the "incoming" path "unref"s an "outgoing state", not > where it's taken? But yeah I can drop it, let me know. > I mean there's a migrate_incoming_ref_outgoing_state() and a migrate_incoming_unref_outgoing_state(), it's obvious enough that they are paired. >> >> > } >> > >> > /** >> > @@ -1888,6 +1913,17 @@ void qmp_migrate_incoming(const char *uri, bool has_channels, >> > return; >> > } >> > >> > + /* >> > + * Making sure MigrationState is available until incoming migration >> > + * completes. >> > + * >> > + * NOTE: QEMU _might_ leak this refcount in some failure paths, but >> > + * that's OK. This is the minimum change we need to at least making >> > + * sure success case is clean on the refcount. We can try harder to >> > + * make it accurate for any kind of failures, but it might be an >> > + * overkill and doesn't bring us much benefit. >> > + */ >> >> Hopefully not any real leak... Let's see what my scripts say about >> it. If it doesn't trigger with migration-test that's fine. > > I remember there could be leaks but only "it fails and dest qemu will quit > immediately" kind of leak. So possible it could trigger with some failure > cases, but not sure whether existing cases will trigger those specific > paths, most failure paths should still get covered. > If this gets flagged by some automated process somewhere it doesn't really matter if it's a reasonable situation because it will get in the way of merging a pull request and I'd rather avoid that. Also Coverity warnings after merge are extremely annoying. This patch looks ok in my testing at least, so: Reviewed-by: Fabiano Rosas queued.