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Re: [PATCH 1/6] migration: Fix and clean up around @tls-authz
From: |
Markus Armbruster |
Subject: |
Re: [PATCH 1/6] migration: Fix and clean up around @tls-authz |
Date: |
Wed, 27 Jan 2021 17:01:57 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
Daniel P. Berrangé <berrange@redhat.com> writes:
> On Thu, Dec 17, 2020 at 02:07:01PM +0100, Markus Armbruster wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>>
>> > On Mon, Dec 14, 2020 at 11:14:34AM +0100, Markus Armbruster wrote:
>> >> Daniel P. Berrangé <berrange@redhat.com> writes:
>> >>
>> >> > On Fri, Nov 13, 2020 at 07:52:31AM +0100, Markus Armbruster wrote:
>> >> >> Commit d2f1d29b95 "migration: add support for a "tls-authz" migration
>> >> >> parameter" added MigrationParameters member @tls-authz. Whereas the
>> >> >> other members aren't really optional (see commit 1bda8b3c695), this
>> >> >> one is genuinely optional: migration_instance_init() leaves it absent,
>> >> >> and migration_tls_channel_process_incoming() passes it to
>> >> >> qcrypto_tls_session_new(), which checks for null.
>> >> >>
>> >> >> Commit d2f1d29b95 has a number of issues, though:
>> >> >>
>> >> >> * When qmp_query_migrate_parameters() copies migration parameters into
>> >> >> its reply, it ignores has_tls_authz, and assumes true instead. When
>> >> >> it is false,
>> >> >>
>> >> >> - HMP info migrate_parameters prints the null pointer (crash bug on
>> >> >> some systems), and
>> >> >>
>> >> >> - QMP query-migrate-parameters replies "tls-authz": "" (because the
>> >> >> QObject output visitor silently maps null pointer to "", which it
>> >> >> really shouldn't).
>> >> >>
>> >> >> The HMP defect was noticed and fixed in commit 7cd75cbdb8
>> >> >> 'migration: use "" instead of (null) for tls-authz'. Unfortunately,
>> >> >> the fix papered over the real bug: it made
>> >> >> qmp_query_migrate_parameters() map null tls_authz to "". It also
>> >> >> dropped the check for has_tls_authz from
>> >> >> hmp_info_migrate_parameters().
>> >> >>
>> >> >> Revert, and fix qmp_query_migrate_parameters() not to screw up
>> >> >> has_tls_authz. No change to HMP. QMP now has "tls-authz" in the
>> >> >> reply only when it's actually present in
>> >> >> migrate_get_current()->parameters. If we prefer to remain
>> >> >> bug-compatible, we should make tls_authz non-optional there.
>> >> >>
>> >> >> * migrate_params_test_apply() neglects to apply tls_authz. Currently
>> >> >> harmless, because migrate_params_check() doesn't care. Fix it
>> >> >> anyway.
>> >> >>
>> >> >> * qmp_migrate_set_parameters() crashes:
>> >> >>
>> >> >> {"execute": "migrate-set-parameters", "arguments": {"tls-authz":
>> >> >> null}}
>> >> >>
>> >> >> Add the necessary rewrite of null to "". For background
>> >> >> information, see commit 01fa559826 "migration: Use JSON null instead
>> >> >> of "" to reset parameter to default".
>> >> >>
>> >> >> Fixes: d2f1d29b95aa45d13262b39153ff501ed6b1ac95
>> >> >> Cc: Daniel P. Berrangé <berrange@redhat.com>
>> >> >> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> >> >> ---
>> >> >> qapi/migration.json | 2 +-
>> >> >> migration/migration.c | 17 ++++++++++++++---
>> >> >> monitor/hmp-cmds.c | 2 +-
>> >> >> 3 files changed, 16 insertions(+), 5 deletions(-)
>> >> >>
>> >> >> diff --git a/qapi/migration.json b/qapi/migration.json
>> >> >> index 3c75820527..688e8da749 100644
>> >> >> --- a/qapi/migration.json
>> >> >> +++ b/qapi/migration.json
>> >> >> @@ -928,7 +928,7 @@
>> >> >> ##
>> >> >> # @MigrationParameters:
>> >> >> #
>> >> >> -# The optional members aren't actually optional.
>> >> >> +# The optional members aren't actually optional, except for
>> >> >> @tls-authz.
>> >> >
>> >> > and tls-hostname and tls-creds.
>> >>
>> >> Really? See [*] below.
>> >>
>> >> >> #
>> >> >> # @announce-initial: Initial delay (in milliseconds) before sending
>> >> >> the
>> >> >> # first announce (Since 4.0)
>> >> >> diff --git a/migration/migration.c b/migration/migration.c
>> >> >> index 3263aa55a9..cad56fbf8c 100644
>> >> >> --- a/migration/migration.c
>> >> >> +++ b/migration/migration.c
>> >> >> @@ -855,9 +855,8 @@ MigrationParameters
>> >> >> *qmp_query_migrate_parameters(Error **errp)
>> >> params->has_tls_creds = true;
>> >> >> params->tls_creds = g_strdup(s->parameters.tls_creds);
>> >> >> params->has_tls_hostname = true;
>> >> >> params->tls_hostname = g_strdup(s->parameters.tls_hostname);
>> >>
>> >> [*] Looks non-optional to me.
>> >
>> > I guess it depends on what you mean by "optional" :-)
>>
>> I meant "non-optional in the value of query-migrate-parameters". The
>> comment were debating applies to that value, and nothing else.
>>
>> > When I say they are all optional, I'm talking about from the POV
>> > of the end users / mgmt who first configures a migration operation.
>> >
>> > tls-creds only needs to be set if you want to enable TLS
>> >
>> > tls-hostname only needs to be set if you need to override the
>> > default hostname used for cert validation.
>> >
>> > tls-authz only needs to be set if you want to enable access
>> > control over migration clients.
>> >
>> > IOW, all three are optional from the POV of configuring a
>> > migration.
>>
>> Understood.
>>
>> > As with many things though, simple theory has turned into
>> > messy reality, by virtue of this previous fixup:
>> >
>> > commit 4af245dc3e6e5c96405b3edb9d75657504256469
>> > Author: Daniel P. Berrangé <berrange@redhat.com>
>> > Date: Wed Mar 15 16:16:03 2017 +0000
>> >
>> > migration: use "" as the default for tls-creds/hostname
>> >
>> > The tls-creds parameter has a default value of NULL indicating
>> > that TLS should not be used. Setting it to non-NULL enables
>> > use of TLS. Once tls-creds are set to a non-NULL value via the
>> > monitor, it isn't possible to set them back to NULL again, due
>> > to current implementation limitations. The empty string is not
>> > a valid QObject identifier, so this switches to use "" as the
>> > default, indicating that TLS will not be used
>> >
>> > The tls-hostname parameter has a default value of NULL indicating
>> > the the hostname from the migrate connection URI should be used.
>> > Again, once tls-hostname is set non-NULL, to override the default
>> > hostname for x509 cert validation, it isn't possible to reset it
>> > back to NULL via the monitor. The empty string is not a valid
>> > hostname, so this switches to use "" as the default, indicating
>> > that the migrate URI hostname should be used.
>> >
>> > Using "" as the default for both, also means that the monitor
>> > commands "info migrate_parameters" / "query-migrate-parameters"
>> > will report existance of tls-creds/tls-parameters even when set
>> > to their default values.
>> >
>> > Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
>> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> > Reviewed-by: Eric Blake <eblake@redhat.com>
>> >
>> > Signed-off-by: Juan Quintela <quintela@redhat.com>
>> >
>> >
>> > I have a nasty feeling that libvirt relies on that last paragraph
>> > to determine whether TLS is supported in QEMU or not too :-( Ideally
>> > we should be able to report their existance, but also report that
>> > they are set to NULL. I guess that could be considered a regression
>> > at this point though.
>> >
>> > So anyway, this explains why we have the wierd behaviour where
>> > querying parameters always reports them as being set.
>>
>> Yes.
>>
>> What do you want me to change in my patch?
>>
>> >> >> - params->has_tls_authz = true;
>> >> >> - params->tls_authz = g_strdup(s->parameters.tls_authz ?
>> >> >> - s->parameters.tls_authz : "");
>> >> >> + params->has_tls_authz = s->parameters.has_tls_authz;
>> >> >
>> >> > I'm kind of confused why has_tls_authz needs to be handled differently
>> >> > from tls_hostname and tls_creds - both of these are optional to
>> >> > the same extent that tls_authz is AFAIR.
>> >>
>> >> I'm kind of confused about pretty much everything around here :)
>> >
>> > So tls_authz was following the wierd precedent used by tls_hostname
>> > and tls_creds in always reporting its own existance, as the empty
>> > string.
>> >
>> >> The patch hunk is part of the revert of flawed commit 7cd75cbdb8. We
>> >> need to revert both parts or none.
>> >>
>> >> One difference between tls_authz and the others is in
>> >> migration_instance_init(): it leaves params->tls_authz null, unlike
>> >> ->tls_hostname and ->tls_creds.
>> >>
>> >> Hmm, it sets ->has_ for none of them. Wrong. If we set ->FOO, we must
>> >> also set ->has_FOO = true, and if we leave ->has_FOO false, we should
>> >> leave ->FOO null.
>> >>
>> >> Another difference is in migration_tls_channel_process_incoming():
>> >> s->parameters.tls_creds must not be null (it's used unchecked in
>> >> migration_tls_get_creds()), while s->parameters.tls_authz may be
>> >> (qcrypto_tls_session_new() checks).
>> >>
>> >> We need to make up our minds what is optional and what isn't.
>> >
>> > So they are all optional in terms of what needs to be set.
>> >
>> > They are all always reported when querying parameters.
>> >
>> > The main difference seems to be that internally we use NULL
>> > as a default for tls_authz, and convert NULL to "" when reporting,
>> > while for tls_creds/tls_hostname we convert NULL to "" immediately
>> > so we always have "" internally.
>> >
>> > Should we instead set tls_authz to "" internally straight away
>> > like we do for tls_creds/tls_hostname, and then make the code
>> > turn "" back into NULL at time of use.
>>
>> I don't know! I'm merely trying to fix a crash bug I ran into :)
>
> Ok, if you don't mind which approach, then I'd vote for making
> migration_instance_init() set tls_authz to "", in common with
> tls_hostname/tls_creds.
>
> Then in migration_tls_channel_process_incoming we can turn the
> "" back into NULL.
>
> That way we'll have consistently used "" internally for all the
> TLS related parameters.
I suffered mental garbage collection during the Christmas break, and can
no longer make heads or tails out of all this.
I might have to drop the series on the floor :(
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