Re: strange tclass in AVCs

[Stephen Smalley <sds@tycho.nsa.gov>]

On 9/19/19 9:02 AM, Ted Toth wrote:
> On Wed, Sep 18, 2019 at 9:18 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
>>
>> On 9/18/19 10:03 AM, Ted Toth wrote:
>>> On Wed, Sep 18, 2019 at 8:53 AM Stephen Smalley <sds@tycho.nsa.gov> wrote:
>>>>
>>>> On 9/18/19 9:35 AM, Ted Toth wrote:
>>>>> I'm seeing things like tclass=context#012 in some AVCs what is this telling me?
>>>>
>>>> Just a guess here, but octal 012 is '\n' aka a newline character, and
>>>> libselinux/src/avc.c:avc_audit() appends a "\n" at the end of the buffer
>>>> before calling avc_log() to log the entire string.  avc_log() will call
>>>> the logging callback, and dbusd does define one, which calls
>>>> audit_log_user_avc_message().  Maybe audit_log_user_avc_message() is
>>>> escaping the newline character in its output as well as appending
>>>> additional data.
>>>>
>>>> I'm a little unclear though on why dbusd is checking a context contains
>>>> permission?
>>>
>>> These appear to only occur when systemd is starting the dbus daemon
>>> and they end up in /var/log/messages not /var/log/audit/audit.log as
>>> I'd expect.
>>
>> Sounds like auditd isn't operational at that point and therefore the
>> output just goes to syslog.
>>
>> Arguably avc_audit() shouldn't be adding a newline at all and that
>> should be handled by the logging callback (or default_selinux_log if no
>> callback is set).  But it has been this way forever, so that would no
>> doubt break some users.  Legacy of when this was a printk/printf.
>>
>>
>>
>>
> 
> FWIW here's the comments from the function dbus uses that calls
> avs_has_perm where the contains check happens. Why dbus policy does
> not allow this is seems like an oversight.

dbusd doesn't check context contains permission AFAIK, only acquire_svc 
and send_msg permissions in the dbus class.  Is that something you 
added?  Or are we picking up some kind of pam module interaction?

> 
> /**
>   * Determine if the SELinux security policy allows the given sender
>   * security context to go to the given recipient security context.
>   * This function determines if the requested permissions are to be
>   * granted from the connection to the message bus or to another
>   * optionally supplied security identifier (e.g. for a service
>   * context).  Currently these permissions are either send_msg or
>   * acquire_svc in the dbus class.
>   *
>   * @param sender_sid source security context
>   * @param override_sid is the target security context.  If SECSID_WILD this will
>   *        use the context of the bus itself (e.g. the default).
>   * @param target_class is the target security class.
>   * @param requested is the requested permissions.
>   * @returns #TRUE if security policy allows the send.
>   */
> #ifdef HAVE_SELINUX
> static dbus_bool_t
> bus_selinux_check (BusSELinuxID        *sender_sid,
>                     BusSELinuxID        *override_sid,
>                     security_class_t     target_class,
>                     access_vector_t      requested,
>                     DBusString          *auxdata)
>