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From:   Steve Grubb <sgrubb@redhat.com>
To:     Tyler Hicks <tyhicks@canonical.com>
Cc:     Paul Moore <paul@paul-moore.com>, linux-kernel@vger.kernel.org,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Will Drewry <wad@chromium.org>, Eric Paris <eparis@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-audit@redhat.com,
        linux-security-module@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v2 3/4] seccomp: Audit attempts to modify the actions_logged sysctl
Date:   Thu, 03 May 2018 17:12:45 -0400
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Organization: Red Hat
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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@redhat.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@redhat.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

-Steve