Re: [PATCH] userns: Default to 'yes' when CONFIG_MEMCG option is enabled

[Javier Martinez Canillas]

Christian Brauner <brauner@xxxxxxxxxx> writes:

> On Fri, May 24, 2024 at 02:33:19PM +0200, Javier Martinez Canillas wrote:
>> Christian Brauner <brauner@xxxxxxxxxx> writes:
>> 
>> Hello Christian,
>> 
>> Thanks a lot for your feedback.
>> 
>> > On Fri, May 24, 2024 at 10:24:16AM +0200, Javier Martinez Canillas wrote:
>> >> The default value for the CONFIG_USER_NS Kconfig symbol changed over time.
>> >> 
>> >> When first was introduced by commit acce292c82d4 ("user namespace: add the
>> >> framework"), the default was 'no'. But then it was changed to 'yes' if the
>> >> CONFIG_NAMESPACES option was enabled, by commit 17a6d4411a4d ("namespaces:
>> >> default all the namespaces to 'yes' when CONFIG_NAMESPACES is selected").
>> >> 
>> >> Then, commit 5673a94c1457 ("userns: Add a Kconfig option to enforce strict
>> >> kuid and kgid type checks") changed the default to 'no' again and selected
>> >> the (now defunct) UIDGID_STRICT_TYPE_CHECKS option.
>> >> 
>> >> This selected option was removed by commit 261000a56b63 ("userns: Remove
>> >> UIDGID_STRICT_TYPE_CHECKS"), but CONFIG_USER_NS default was left to 'no'.
>> >> 
>> >> Finally, the commit e11f0ae388f2 ("userns: Recommend use of memory control
>> >> groups") added to the Kconfig symbol's help text a recommendation that the
>> >> memory control groups should be used, to limit the amount of memory that a
>> >> user who can create user namespaces can consume.
>> >> 
>> >> Looking at the changes' history, a default to 'yes' when the CONFIG_MEMCG
>> >> option is enabled seems like a sane thing to do. Specially since systemd
>> >> requires user namespaces support for services that use the PrivateUsers=
>> >> property in their unit files (e.g: the UPower daemon).
>> >
>> > Fyi, user namespaces are an entirely optional feature in systemd and it
>> > gracefully falls back if they are not available with PrivateUsers= set.
>> > If that isn't the case then it's a bug in systemd with PrivateUsers=
>> > handling and should be reported.
>> >
>> 
>> Interesting, it definitely failed for me:
>> 
>> $ systemctl status upower
>> ● upower.service - Daemon for power management
>>      Loaded: loaded (/lib/systemd/system/upower.service; disabled; vendor preset: enabled)
>>      Active: failed (Result: exit-code) since Fri 2024-05-24 12:23:49 UTC; 34s ago
>>        Docs: man:upowerd(8)
>>     Process: 390 ExecStart=/usr/libexec/upowerd (code=exited, status=217/USER)
>>    Main PID: 390 (code=exited, status=217/USER)
>>         CPU: 122ms
>> 
>> May 24 12:23:49 igep systemd[1]: upower.service: Scheduled restart job, restart counter is at 5.
>> May 24 12:23:49 igep systemd[1]: Stopped Daemon for power management.
>> May 24 12:23:49 igep systemd[1]: upower.service: Start request repeated too quickly.
>> May 24 12:23:49 igep systemd[1]: upower.service: Failed with result 'exit-code'.
>> May 24 12:23:49 igep systemd[1]: Failed to start Daemon for power management.
>> 
>> $ journalctl -u upower
>> May 24 12:23:49 igep systemd[1]: Starting Daemon for power management...
>> May 24 12:23:49 igep systemd[404]: upower.service: Failed to set up user namespacing: Invalid argument
>> May 24 12:23:49 igep systemd[404]: upower.service: Failed at step USER spawning /usr/libexec/upowerd: Invalid argument
>> May 24 12:23:49 igep systemd[1]: upower.service: Main process exited, code=exited, status=217/USER
>> May 24 12:23:49 igep systemd[1]: upower.service: Failed with result 'exit-code'.
>> May 24 12:23:49 igep systemd[1]: Failed to start Daemon for power management.
>> May 24 12:23:49 igep systemd[1]: upower.service: Scheduled restart job, restart counter is at 1.
>> May 24 12:23:49 igep systemd[1]: Stopped Daemon for power management.
>> 
>> That lead me to https://gitlab.freedesktop.org/upower/upower/-/issues/104
>> and finally to systemd's README:
>> 
>> https://github.com/systemd/systemd/blob/main/README#L89C22-L89C34 
>> 
>> But I'll investigate more if is upower or systemd to be blamed here...
>> 
>> > But specifically to you change, afair CONFIG_MEMCG and userns are
>> > unrelated so tying them together like this in the kconfig seems
>> > misguided.
>> >
>> 
>> Yes, but the config USER_NS help text already tieds them toghether:
>> 
>> 	help
>> 	  This allows containers, i.e. vservers, to use user namespaces
>> 	  to provide different user info for different servers.
>> 
>> 	  When user namespaces are enabled in the kernel it is
>> 	  recommended that the MEMCG option also be enabled and that
>
> But then the patch your patch is the wrong way around and you should
> make CONFIG_USER_NS select CONFIG_MEMCG. IOW, if you do userns, do memcg
> but _not_ if you do memcg, do userns.
>

You are right.

>> 	  user-space use the memory control groups to limit the amount
>> 	  of memory a memory unprivileged users can use.
>> 
>> And as mentioned in the commit message, it seems to be the reason why the
>> default for this Kconfig symbol is no. Maybe I misunderstood though or do
>> you think that could be switched unconditionally to 'default y' ?
>
> No, definitely not.
>

Ok.

>> 
>> Or is there a reason to be the only namespace to be default to no instead
>> of yes? Specially since important system services are trying to use it.
>
> Yes, it has a lot more security implications than all of the other ones
> and makes them available to unprivileged users by default. So that
> definitely requires a conscious choice.
>

Got it. Thanks for the explanation. Maybe the option help text could be a
little bit more verbose about it? Since it wasn't clear, at least to me.

Let's discard this patch then and sorry for the noise.

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat