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&gt; 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&gt; 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&gt; > > > > --- > > > > 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