linux-kernel - Re: [RFC v7 10/41] richacl: Permission check algorithm

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Message-ID: <CAHpGcMJofUKKumcUe=6LOFZrVcJRM5APchbCQpfQzbMK7N1FYg@mail.gmail.com>
Date:	Sat, 12 Sep 2015 00:12:16 +0200
From:	Andreas Grünbacher <andreas.gruenbacher@...il.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Andreas Gruenbacher <agruenba@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Linux API Mailing List <linux-api@...r.kernel.org>,
	linux-cifs@...r.kernel.org,
	LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [RFC v7 10/41] richacl: Permission check algorithm

2015-09-11 23:16 GMT+02:00 J. Bruce Fields <bfields@...ldses.org>:
> On Sat, Sep 05, 2015 at 12:27:05PM +0200, Andreas Gruenbacher wrote:
>> +             /*
>> +              * Apply the group file mask to entries other than owner@ and
>> +              * everyone@ or user entries matching the owner.  This ensures
>> +              * that we grant the same permissions as the acl computed by
>> +              * richacl_apply_masks().
>> +              *
>> +              * Without this restriction, the following richacl would grant
>> +              * rw access to processes which are both the owner and in the
>> +              * owning group, but not to other users in the owning group,
>> +              * which could not be represented without masks:
>> +              *
>> +              *  owner:rw::mask
>> +              *  group@:rw::allow
>> +              */
>> +             if ((acl->a_flags & RICHACL_MASKED) && richace_is_allow(ace))
>> +                     ace_mask &= acl->a_group_mask;
>
> I'm having trouble understanding this.  I think the problem is that I
> don't really understand the notation in your example.  Is a_group_mask
> zero in that example?  I think it must be, in which case, OK I think I
> get it.

Yes. I'm not sure if the example becomes easier to understand when the
empty group mask and perhaps also the other mask is included.

> (Though I still have to think about it a little more to convince myself
> that richacl_apply_masks() always gets the same result.)

I have tried to break the algorithm into digestible pieces. Do you see
another way to make things easier to understand?

Thanks,
Andreas
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