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

[Richard Guy Briggs]

On 2019-11-08 12:49, Paul Moore wrote:
> On Thu, Oct 24, 2019 at 5:23 PM Richard Guy Briggs <rgb at redhat.com> wrote:
> > On 2019-10-10 20:38, Paul Moore wrote:
> > > On Fri, Sep 27, 2019 at 8:52 AM Neil Horman <nhorman at tuxdriver.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 at redhat.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.

It was more of a concern for total system resources, primarily memory,
but this is self-limiting and an arbitrary concern.

The other limit of depth of nesting has different concerns that arise
depending on how reporting is done.

> > 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.

Ok, I've dropped it.  There are mitigations in place for large numbers
of contids and it can be limited later without breaking anything.

> > 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.

I've switched to using an rcu read lock on the list traversal and
spin-lock on list update.

> paul moore

- RGB

--
Richard Guy Briggs <rgb at redhat.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