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

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Message-ID: <20191024212335.y4ou7g4tsxnotvnk@madcap2.tricolour.ca>
Date:   Thu, 24 Oct 2019 17:23:35 -0400
From:   Richard Guy Briggs <rgb@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Neil Horman <nhorman@...driver.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 2019-10-10 20:38, Paul Moore wrote:
> 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.

Can you separate and clarify the concerns here?

I plan to move this patch to the end of the patchset and make it
optional, possibly adding a tuning mechanism.  Like the migration from
/proc to netlink for loginuid/sessionid/contid/capcontid, this was Eric
Biederman's concern and suggested mitigation.

As for the first issue of the bucket chain length traversal while
holding the list spin-lock, would you prefer to use the rcu lock to
traverse the list and then only hold the spin-lock when modifying the
list, and possibly even make the spin-lock more fine-grained per list?

> paul moore

- RGB

--
Richard Guy Briggs <rgb@...hat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635