netdev - Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin

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Date:   Sat, 26 Jan 2019 01:59:12 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        jakub.kicinski@...ronome.com,
        Network Development <netdev@...r.kernel.org>,
        kernel-team@...com, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v4 bpf-next 1/9] bpf: introduce bpf_spin_lock

On Sat, Jan 26, 2019 at 1:43 AM Jann Horn <jannh@...gle.com> wrote:
> On Sat, Jan 26, 2019 at 12:44 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Fri, Jan 25, 2019 at 02:51:12PM -0800, Paul E. McKenney wrote:
> > > > >
> > > > > So no more than (say) 100 milliseconds?
> > > >
> > > > Depends on RLIMIT_MEMLOCK and on how hard userspace is trying to make
> > > > things slow, I guess - if userspace manages to create a hashtable,
> > > > with a few dozen megabytes in size, with worst-case assignment of
> > > > elements to buckets (everything in a single bucket), every lookup call
> > > > on that bucket becomes a linked list traversal through a list that
> > > > must be stored in main memory because it's too big for the CPU caches.
> > > > I don't know into how much time that translates.
> > >
> > > So perhaps you have a candidate BPF program for the RCU CPU stall warning
> > > challenge, then.  ;-)
> >
> > I'd like to see one that can defeat jhash + random seed.
>
> Assuming that the map isn't created by root with BPF_F_ZERO_SEED:
>
> The dumb approach would be to put things into the map, try to measure
> via timing/sidechannel whether you got collisions, and then keep
> trying different keys, and keep them if the timing indicates a
> collision. That'd probably be pretty slow and annoying though. Two
> years ago, I implemented something similar to leak information about
> virtual addresses from Firefox by measuring hash bucket collisions
> from JavaScript (but to be fair, it was easier there because you can
> resize the hash table):
> https://thejh.net/misc/firefox-cve-2016-9904-and-cve-2017-5378-bugreport
>
> But I think there's an easier way, too: The jhash seed is just 32
> bits, and AFAICS the BPF API leaks information about that seed through
> BPF_MAP_GET_NEXT_KEY: Stuff two random keys into the hash table, run
> BPF_MAP_GET_NEXT_KEY with attr->key==NULL, and see which key is
> returned. Do that around 32 times, and you should have roughly enough
> information to bruteforce the jhash seed? Recovering the seed should
> then be relatively quick, 2^32 iterations of a fast hash don't take
> terribly long.
>
> That said, I don't think this is interesting enough to spend the time
> necessary to implement it. :P

Oh, and actually, you can probably also detect a collision in a simpler way:

 - insert A
 - insert B
 - query BPF_MAP_GET_NEXT_KEY
 - delete A
 - delete B
 - insert B
 - insert A
 - query BPF_MAP_GET_NEXT_KEY
 - delete A
 - delete B

If the two BPF_MAP_GET_NEXT_KEY queries return the same result, A and
B are in different buckets; if they return different results, A and B
are in the same bucket, I think?