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(a)redhat.com> wrote:
On 2019-10-10 20:38, Paul Moore wrote:
> On Fri, Sep 27, 2019 at 8:52 AM Neil Horman <nhorman(a)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(a)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.
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