Re: Hundreds of null PATH records for *init_module syscall audit logs

[Richard Guy Briggs <rgb@redhat.com>]

On 2017-03-03 19:22, Paul Moore wrote:
> On Fri, Mar 3, 2017 at 4:14 PM, Richard Guy Briggs <rgb@redhat.com> wrote:
> > On 2017-02-28 23:15, Steve Grubb wrote:
> >> On Tuesday, February 28, 2017 10:37:04 PM EST Richard Guy Briggs wrote:
> >> > Sorry, I forgot to include Cc: in this cover letter for context to the 4
> >> > alt patches.
> >> >
> >> > On 2017-02-28 22:15, Richard Guy Briggs wrote:
> >> > > The background to this is:
> >> > >   https://github.com/linux-audit/audit-kernel/issues/8
> >> > >
> >> > > In short, audit SYSCALL records for *init_module were occasionally
> >> > > accompanied by hundreds to thousands of null PATH records.
> >> > >
> >> > > I chatted with Al Viro and Eric Paris about this Friday afternoon and
> >> > > they seemed to vaguely recall this issue and didn't have any solid
> >> > > recommendations as to what was the right thing to do (other than the
> >> > > same suggestion from both that I won't print here).
> >> > >
> >> > > It was reproducible on a number of vintages of distributions with
> >> > > default kernels, but triggering on very few of the many modules loaded
> >> > > at boot time.  It was reproduced with fs-nfs4 and nfsv4 modules on
> >> > > tracefs, but there are reports of it also happening with debugfs.  It
> >> > > was triggering only in __audit_inode_child with a parent that was not
> >> > > found in the task context's audit names_list.
> >> > >
> >> > > I have four potential solutions listed in my order of preference and I'd
> >> > > like to get some feedback about which one would be the most acceptable.
> >>
> >> 0.5 - Notice that we are in *init_module & delete_module and inhibit
> >> generation of any record type except SYSCALL and KERN_MODULE ? There are some
> >> classification routines for -F perms=wrxa that might be used to create a new
> >> class for loading/deleting modules that sets a flag that we use to suppress
> >> some record types.
> >
> > Ok, I was partially able to do this.
> >
> > If I try and catch it in audit_log_start() which is the common point for
> > all the record types to be able to limit to just SYSCALL and
> > KERN_MODULE, there will already be a linked list of hundreds to
> > thousands of audit_names and will still print a non-zero items count in
> > the SYSCALL record.  This also sounds like a potentially lazy way to
> > deal with other record spam (like setuid BRPM_FCAPS).
> >
> > If I catch it in __audit_inode_child in the same place as I caught the
> > filesystem type, it is effective for only the PATH record, which is all
> > that is a problem at the moment.
> >
> > It touches nine arch-related files, which is a lot more disruptive than
> > I was hoping.
> 
> Blocking PATH record on creation based on syscall *really* seems like
> a bad/dangerous idea.  If we want to block all these tracefs/debugfs
> records, let's just block the fs.  Although as of right now I'm not a
> fan of blocking anything.

I agree.  What makes me leery of this approach is if a kernel module in
turn accesses directly other files, or bypasses the load_module call to
load another module from a file and avoids logging.

> paul moore

- RGB

--
Richard Guy Briggs <rgb@redhat.com>
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635