linux-kernel - Re: [PATCH] audit: catch errors from audit_filter

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Message-ID: <20160628172921.GD30821@madcap2.tricolour.ca>
Date:	Tue, 28 Jun 2016 13:29:21 -0400
From:	Richard Guy Briggs <rgb@...hat.com>
To:	Paul Moore <paul@...l-moore.com>
Cc:	linux-audit@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] audit: catch errors from audit_filter_rules field checks

On 2016-06-16 17:07, Paul Moore wrote:
> On Tue, Jun 14, 2016 at 5:03 PM, Richard Guy Briggs <rgb@...hat.com> wrote:
> > In the case of an error returned from a field check in an audit filter
> > syscall rule, it is treated as a match and the rule action is honoured.
> >
> > This could cause a rule with a default of NEVER and an selinux field
> > check error to avoid logging.
> >
> > Recommend matching with an action of ALWAYS to catch malicious abuse of
> > this bug.  The downside of this approach is it could DoS the audit
> > subsystem.
> 
> I understand your concern about the DoS, but in reality it is no worse
> than if no audit filter rules were configured, yes?

Are you thinking of audit_filter_type which has now been merged with
audit_filter_user?

This is audit_filter_rules, which is used by syscalls with a much
broader choice of selectors.

If there are no rules set, there are no messages recorded other than
AVCs.  If a rule was configured and an error occurred in one of the
SELinux checks, it would match and not report.  I'd argue it should fail
safe and report.

> > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > ---
> >  kernel/auditsc.c |    4 ++++
> >  1 files changed, 4 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index 71e14d8..6123672 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -683,6 +683,10 @@ static int audit_filter_rules(struct task_struct *tsk,
> >                 }
> >                 if (!result)
> >                         return 0;
> > +               if (result < 0) {
> > +                       *state = AUDIT_RECORD_CONTEXT;
> > +                       return 1;
> > +               }
> >         }
> >
> >         if (ctx) {
> 
> -- 
> paul moore
> www.paul-moore.com

- RGB

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