linux-kernel - Re: [PATCH ghak90 V7 06/21] audit: contid limit of 32k imposed to avoid DoS

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Message-ID: <CAHC9VhRbSUCB0OZorC4+y+5uJDR5uMXdRn2LOTYGu2gcFJSrcA@mail.gmail.com>
Date:   Thu, 10 Oct 2019 20:38:58 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Neil Horman <nhorman@...driver.com>
Cc:     Richard Guy Briggs <rgb@...hat.com>,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        sgrubb@...hat.com, omosnace@...hat.com, dhowells@...hat.com,
        simo@...hat.com, Eric Paris <eparis@...isplace.org>,
        Serge Hallyn <serge@...lyn.com>, ebiederm@...ssion.com,
        Dan Walsh <dwalsh@...hat.com>, mpatel@...hat.com
Subject: Re: [PATCH ghak90 V7 06/21] audit: contid limit of 32k imposed to
 avoid DoS

On Fri, Sep 27, 2019 at 8:52 AM Neil Horman <nhorman@...driver.com> wrote:
> On Wed, Sep 18, 2019 at 09:22:23PM -0400, Richard Guy Briggs wrote:
> > Set an arbitrary limit on the number of audit container identifiers to
> > limit abuse.
> >
> > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > ---
> >  kernel/audit.c | 8 ++++++++
> >  kernel/audit.h | 4 ++++
> >  2 files changed, 12 insertions(+)
> >
> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index 53d13d638c63..329916534dd2 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c

...

> > @@ -2465,6 +2472,7 @@ int audit_set_contid(struct task_struct *task, u64 contid)
> >                               newcont->owner = current;
> >                               refcount_set(&newcont->refcount, 1);
> >                               list_add_rcu(&newcont->list, &audit_contid_hash[h]);
> > +                             audit_contid_count++;
> >                       } else {
> >                               rc = -ENOMEM;
> >                               goto conterror;
> > diff --git a/kernel/audit.h b/kernel/audit.h
> > index 162de8366b32..543f1334ba47 100644
> > --- a/kernel/audit.h
> > +++ b/kernel/audit.h
> > @@ -219,6 +219,10 @@ static inline int audit_hash_contid(u64 contid)
> >       return (contid & (AUDIT_CONTID_BUCKETS-1));
> >  }
> >
> > +extern int audit_contid_count;
> > +
> > +#define AUDIT_CONTID_COUNT   1 << 16
> > +
>
> Just to ask the question, since it wasn't clear in the changelog, what
> abuse are you avoiding here?  Ostensibly you should be able to create as
> many container ids as you have space for, and the simple creation of
> container ids doesn't seem like the resource strain I would be concerned
> about here, given that an orchestrator can still create as many
> containers as the system will otherwise allow, which will consume
> significantly more ram/disk/etc.

I've got a similar question.  Up to this point in the patchset, there
is a potential issue of hash bucket chain lengths and traversing them
with a spinlock held, but it seems like we shouldn't be putting an
arbitrary limit on audit container IDs unless we have a good reason
for it.  If for some reason we do want to enforce a limit, it should
probably be a tunable value like a sysctl, or similar.

--
paul moore
www.paul-moore.com