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

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Date:   Fri, 8 Nov 2019 12:49:23 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Richard Guy Briggs <rgb@...hat.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 Thu, Oct 24, 2019 at 5:23 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> 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?

"Why are you doing this?" is about as simple as I can pose the question.

> 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.

Okay, let's just drop it.  I *really* don't like this approach of
tossing questionable stuff at the end of the patchset; I get why you
are doing it, but I think we really need to focus on keeping this
changeset small.  If the number of ACIDs (heh) become unwieldy the
right solution is to improve the algorithms/structures, if we can't do
that for some reason, *then* we can fall back to a limiting knob in a
latter release.

> 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?

Until we have a better idea of how this is going to be used, I think
it's okay for now.  It's also internal to the kernel so we can change
it at any time.  My comments about the locking/structs was only to try
and think of some reason why one might want to limit the number of
ACIDs since neither you or Eric provided any reasoning that I could
see.

-- 
paul moore
www.paul-moore.com