linux-kernel - Re: [PATCH v2 3/4] seccomp: Audit attempts to modify the actions

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Date:   Thu, 3 May 2018 17:36:18 -0500
From:   Tyler Hicks <tyhicks@...onical.com>
To:     Steve Grubb <sgrubb@...hat.com>, Paul Moore <paul@...l-moore.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Eric Paris <eparis@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-audit@...hat.com,
        linux-security-module@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 3/4] seccomp: Audit attempts to modify the
 actions_logged sysctl

On 05/03/2018 04:12 PM, Steve Grubb wrote:
> On Thursday, May 3, 2018 4:51:36 PM EDT Tyler Hicks wrote:
>> On 05/03/2018 03:48 PM, Paul Moore wrote:
>>> On Thu, May 3, 2018 at 4:42 PM, Steve Grubb <sgrubb@...hat.com> wrote:
>>>> On Thursday, May 3, 2018 4:18:26 PM EDT Paul Moore wrote:
>>>>> On Wed, May 2, 2018 at 2:18 PM, Steve Grubb <sgrubb@...hat.com> wrote:
>>>>>> On Wednesday, May 2, 2018 11:53:19 AM EDT Tyler Hicks wrote:
>>>>>>> The decision to log a seccomp action will always be subject to the
>>>>>>> value of the kernel.seccomp.actions_logged sysctl, even for processes
>>>>>>> that are being inspected via the audit subsystem, in an upcoming
>>>>>>> patch.
>>>>>>> Therefore, we need to emit an audit record on attempts at writing to
>>>>>>> the
>>>>>>> actions_logged sysctl when auditing is enabled.
>>>>>>>
>>>>>>> This patch updates the write handler for the actions_logged sysctl to
>>>>>>> emit an audit record on attempts to write to the sysctl. Successful
>>>>>>> writes to the sysctl will result in a record that includes a
>>>>>>> normalized
>>>>>>> list of logged actions in the "actions" field and a "res" field equal
>>>>>>> to
>>>>>>> 0. Unsuccessful writes to the sysctl will result in a record that
>>>>>>> doesn't include the "actions" field and has a "res" field equal to 1.
>>>>>>>
>>>>>>> Not all unsuccessful writes to the sysctl are audited. For example,
>>>>>>> an
>>>>>>> audit record will not be emitted if an unprivileged process attempts
>>>>>>> to
>>>>>>> open the sysctl file for reading since that access control check is
>>>>>>> not
>>>>>>> part of the sysctl's write handler.
>>>>>>>
>>>>>>> Below are some example audit records when writing various strings to
>>>>>>> the
>>>>>>> actions_logged sysctl.
>>>>>>>
>>>>>>> Writing "not-a-real-action", when the kernel.seccomp.actions_logged
>>>>>>> sysctl previously was "kill_process kill_thread trap errno trace
>>>>>>> log",
>>>>>>>
>>>>>>> emits this audit record:
>>>>>>>  type=CONFIG_CHANGE msg=audit(1525275273.537:130): op=seccomp-logging
>>>>>>>  old-actions=kill_process,kill_thread,trap,errno,trace,log res=0
>>>>>>>
>>>>>>> If you then write "kill_process kill_thread errno trace log", this
>>>>>>> audit
>>>>>>>
>>>>>>> record is emitted:
>>>>>>>  type=CONFIG_CHANGE msg=audit(1525275310.208:136): op=seccomp-logging
>>>>>>>  actions=kill_process,kill_thread,errno,trace,log
>>>>>>>  old-actions=kill_process,kill_thread,trap,errno,trace,log res=1
>>>>>>>
>>>>>>> If you then write the string "log log errno trace kill_process
>>>>>>> kill_thread", which is unordered and contains the log action twice,
>>>>>>>
>>>>>>> it results in the same actions value as the previous record:
>>>>>>>  type=CONFIG_CHANGE msg=audit(1525275325.613:142): op=seccomp-logging
>>>>>>>  actions=kill_process,kill_thread,errno,trace,log
>>>>>>>  old-actions=kill_process,kill_thread,errno,trace,log res=1
>>>>>>>
>>>>>>> No audit records are generated when reading the actions_logged
>>>>>>> sysctl.
>>>>>>
>>>>>> ACK for the format of the records.
>>>>>
>>>>> I just wanted to clarify the record format with you Steve ... the
>>>>> "actions" and "old-actions" fields may not be included in the record
>>>>> in cases where there is an error building the action value string, are
>>>>> you okay with that or would you prefer the fields to always be
>>>>> included but with a "?" for the value?
>>>>
>>>> A ? would be more in line with how other things are handled.
>>>
>>> That's what I thought.
>>>
>>> Would you mind putting together a v3 Tyler? :)
>>
>> To be clear, "?" is only to be used when the call to
>> seccomp_names_from_actions_logged() fails, right?
> 
> Yes and that is a question mark with no quotes in the audit record.
> 
>> If the sysctl write fails for some other reason, such as when an invalid
>> action name is specified, can you confirm that you still want *no*
>> "actions" field, 
> 
> Its best that fields do not disappear. In the case of invalid input, you can 
> just leave the new value as ? so that nothing malicious can be injected into 
> the logs
> 
>> the "old-actions" field to be the value prior to attempting the update to
>> the sysctl, and res to be 0?
> 
> Yes

I came up with one more question after hitting a corner case while testing.

It is valid to write an empty string to the sysctl. If the sysctl was
set to "errno" and then later set to "", you'd see this with the current
revision:

 type=CONFIG_CHANGE msg=audit(1525385824.643:173): op=seccomp-logging
 actions= old-actions=errno res=1

Is that what you want or should the value of the "actions" field be
something be something like this:

 actions=(none)

Tyler



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