linux-kernel - Re: [PATCH v2 1/3] capabilities: Introduce CAP_CHECKPOINT

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Message-ID: <37b47c7d-a24e-c453-5168-c383e6c36c9f@schaufler-ca.com>
Date:   Wed, 10 Jun 2020 08:41:29 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Andrei Vagin <avagin@...il.com>,
        Christian Brauner <christian.brauner@...ntu.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Michał Cłapiński <mclapinski@...gle.com>,
        Kamil Yurtsever <kyurtsever@...gle.com>,
        Dirk Petersen <dipeit@...il.com>,
        Christine Flood <chf@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Arnd Bergmann <arnd@...db.de>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
        Eric Paris <eparis@...isplace.org>,
        Jann Horn <jannh@...gle.com>, linux-fsdevel@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v2 1/3] capabilities: Introduce CAP_CHECKPOINT_RESTORE


On 6/10/2020 12:59 AM, Andrei Vagin wrote:
> On Tue, Jun 09, 2020 at 06:14:27PM +0200, Christian Brauner wrote:
>> On Tue, Jun 09, 2020 at 09:06:27AM -0700, Andrei Vagin wrote:
>>> On Tue, Jun 09, 2020 at 09:44:22AM +0200, Christian Brauner wrote:
>>>> On Mon, Jun 08, 2020 at 08:42:21PM -0700, Andrei Vagin wrote:
> ...
>>>>> PTRACE_O_SUSPEND_SECCOMP is needed for C/R and it is protected by
>>>>> CAP_SYS_ADMIN too.
>>>> This is currently capable(CAP_SYS_ADMIN) (init_ns capable) why is it
>>>> safe to allow unprivileged users to suspend security policies? That
>>>> sounds like a bad idea.
> ...
>>> I don't suggest to remove or
>>> downgrade this capability check. The patch allows all c/r related
>>> operations if the current has CAP_CHECKPOINT_RESTORE.
>>>
>>> So in this case the check:
>>>      if (!capable(CAP_SYS_ADMIN))
>>>              return -EPERM;
>>>
>>> will be converted in:
>>>      if (!capable(CAP_SYS_ADMIN) && !capable(CAP_CHECKPOINT_RESTORE))
>>>              return -EPERM;
>> Yeah, I got that but what's the goal here? Isn't it that you want to
>> make it safe to install the criu binary with the CAP_CHECKPOINT_RESTORE
>> fscap set so that unprivileged users can restore their own processes
>> without creating a new user namespace or am I missing something? The
>> use-cases in the cover-letter make it sound like that's what this is
>> leading up to:
>>>>>> * Checkpoint/Restore in an HPC environment in combination with a resource
>>>>>>   manager distributing jobs where users are always running as non-root.
>>>>>>   There is a desire to provide a way to checkpoint and restore long running
>>>>>>   jobs.
>>>>>> * Container migration as non-root
>>>>>> * We have been in contact with JVM developers who are integrating
>>>>>>   CRIU into a Java VM to decrease the startup time. These checkpoint/restore
>>>>>>   applications are not meant to be running with CAP_SYS_ADMIN.
>> But maybe I'm just misunderstanding crucial bits (likely (TM)).
> I think you understand this right. The goal is to make it possible to
> use C/R functionality for unprivileged processes.

Y'all keep saying "unprivileged processes" when you mean
"processes with less than root privilege". A process with
CAP_CHECKPOINT_RESTORE *is* a privileged process. It would
have different privilege from a process with CAP_SYS_ADMIN
(the current case) but is not "unprivileged".

>  And for me, here are
> two separate tasks. The first one is how to allow unprivileged users to
> use C/R from the root user namespace. This is what we discuss here.
>
> And another one is how to allow to use C/R functionality from a non-root
> user namespaces. The second task is about downgrading capable to
> ns_capable for map_files and PTRACE_O_SUSPEND_SECCOMP.
>
> Thanks,
> Andrei