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

From: Steve Grubb <[email-masked]>
Date: Tue, 17 Dec 2019 14:25:29 -0500

On Tuesday, December 17, 2019 1:45:41 PM EST Richard Guy Briggs wrote:
> On 2019-11-08 12:49, Paul Moore wrote:
> > On Thu, Oct 24, 2019 at 5:23 PM Richard Guy Briggs <[email-masked]> 
wrote:
> > > On 2019-10-10 20:38, Paul Moore wrote:
> > > > On Fri, Sep 27, 2019 at 8:52 AM Neil Horman <[email-masked]> 
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 <[email-masked]>
> > > > > > ---
> > > > > > 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.

Well, there is a limit on the audit record size. So, whatever is being sent 
in the record plus the size of the timestamp deducted from 
MAX_AUDIT_MESSAGE_LENGTH (8970) is the limit. That can be divided by however 
many ID's fit in that space and you have the real limit.

-Steve

-Steve