linux-kernel - Re: kdbus_proc_permission (Re: [GIT PULL] kdbus updates for Greg)

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Message-ID: <CALCETrVd2rEAuBkCML+ESHLhWT90fk8MJ-S580c84xzBuugthg@mail.gmail.com>
Date:	Mon, 31 Aug 2015 08:37:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: kdbus_proc_permission (Re: [GIT PULL] kdbus updates for Greg)

On Mon, Aug 24, 2015 at 2:52 AM, David Herrmann <dh.herrmann@...il.com> wrote:
> Hi
>
> On Mon, Aug 10, 2015 at 4:42 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> I spotted this:
>>
>> +/**
>> + * kdbus_proc_permission() - check /proc permissions on target pid
>> + * @pid_ns:            namespace we operate in
>> + * @cred:              credentials of requestor
>> + * @target:            target process
>> + *
>> + * This checks whether a process with credentials @cred can access information
>> + * of @target in the namespace @pid_ns. This tries to follow /proc permissions,
>> + * but is slightly more restrictive.
>> + *
>> + * Return: The /proc access level (KDBUS_META_PROC_*) is returned.
>> + */
>> +static unsigned int kdbus_proc_permission(const struct pid_namespace *pid_ns,
>> +                                         const struct cred *cred,
>> +                                         struct pid *target)
>>
>> That code ended up in a pull request, although AFAICT it was never in
>> any patch email sent to me or to any public mailing list.  I suspect
>> it was at least partially a response to one of my old reviews.
>
> Exactly. It's an attempt to model metadata access similar to /proc
> access (thus, access to kdbusfs implies access to procfs, but not vice
> versa (nor any implication on hidepid)).

I haven't looked at what calls this function, but I guess the model is
inaccurate and errs in the too-permissive direction.

Fedora actually labels inodes in /proc (ls -Zd /proc/???, for
example), but I don't know what it does with those labels.

>
>> I haven't checked the context in which it's used, but in order for
>> kdbus_proc_permission to do what it claims to do, it appears to be
>> missing calls to security_inode_permission and
>> security_file_permission.
>
> Both are expected to be added by lsm patches (both hooks you mentioned
> are empty if no lsm is selected).

Will that mean that existing MAC policies stop being fully enforced
(in effect) if kdbus is installed?

--Andy
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