netdev - Re: [PATCH 1/3] bpf: allow zero-initializing hash map seed

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Message-ID: <CACAyw9-JcORMGDb3wm-E4VYmjVmvWqB5vVh+=qTvXmpFWye_QA@mail.gmail.com>
Date:   Fri, 5 Oct 2018 15:21:16 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     jannh@...gle.com
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 1/3] bpf: allow zero-initializing hash map seed

On Fri, 5 Oct 2018 at 15:12, Jann Horn <jannh@...gle.com> wrote:
>
> On Fri, Oct 5, 2018 at 9:42 AM Lorenz Bauer <lmb@...udflare.com> wrote:
> > On Tue, 2 Oct 2018 at 21:00, Jann Horn <jannh@...gle.com> wrote:
> > >
> > > If this is for testing only, you can slap a capable(CAP_SYS_ADMIN)
> > > check in here, right? I doubt it matters, but I don't really like
> > > seeing something like this exposed to unprivileged userspace just
> > > because you need it for kernel testing.
> >
> > That would mean all tests have to run as root / with CAP_SYS_ADMIN
> > which isn't ideal.
>
> This patch basically means that it becomes easier for a local user to
> construct a BPF hash table that has all of its values stuffed into a
> single hash bucket, correct? Which makes it easier to create a BPF
> program that generates unusually large RCU stalls by performing ~40000
> BPF map lookups, each of which has to walk through the entire linked
> list of the hash map bucket? I dislike exposing something like that to
> unprivileged userspace.

That's a good point, for which I don't have an answer. You could argue that
this was the status quo until the seed was randomised, so it seems
like this hasn't been a worry so far. Should it be going forward?

> And if you want to run the whole BPF test suite with all its tests,
> don't you already need root privileges? Or is this a different test
> suite?

No, I'm thinking about third parties that want to test their own BPF.
If you enable unprivileged BPF you can use BPF_PROG_TEST_RUN to
test your programs without root, if I'm not mistaken.
-- 
Lorenz Bauer  |  Systems Engineer
25 Lavington St., London SE1 0NZ

www.cloudflare.com