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

[Jessica Yu]

+++ Richard Guy Briggs [06/03/17 16:49 -0500]:
> On 2017-03-03 19:22, Paul Moore wrote:
> > On Fri, Mar 3, 2017 at 4:14 PM, Richard Guy Briggs <rgb@xxxxxxxxxx> 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.

AFAIK load_module is *the* entry point for module loading, it is where
all the setup occurs in order for a module to be properly set up and
registered in our internal data structures (e.g the global modules
list). If a module wants another module loaded, it can request for it
to be loaded via request_module(), which punts the request to modprobe
in userspace to load the module in question, but I'm not sure if
that's at all related to this null PATH record issue.

Jessica