netdev - Re: [PATCH bpf-next v6 11/11] selftests/bpf: Add test for race in btf_try_get

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Date:   Fri, 7 Jan 2022 12:52:36 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        netfilter-devel@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...dia.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next v6 11/11] selftests/bpf: Add test for race in
 btf_try_get_module

On Fri, Jan 07, 2022 at 01:09:04AM IST, Andrii Nakryiko wrote:
> On Thu, Jan 6, 2022 at 1:04 AM Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
> >
> > On Wed, Jan 05, 2022 at 11:50:33AM IST, Alexei Starovoitov wrote:
> > > On Sun, Jan 02, 2022 at 09:51:15PM +0530, Kumar Kartikeya Dwivedi wrote:
> > > > This adds a complete test case to ensure we never take references to
> > > > modules not in MODULE_STATE_LIVE, which can lead to UAF, and it also
> > > > ensures we never access btf->kfunc_set_tab in an inconsistent state.
> > > >
> > > > The test uses userfaultfd to artifically widen the race.
> > >
> > > Fancy!
> > > Does it have to use a different module?
> > > Can it be part of bpf_testmod somehow?
> >
> > I was thinking of doing it with bpf_testmod, but then I realised it would be a
> > problem with parallel mode of test_progs, where another selftest in parallel may
> > rely on bpf_testmod (which this test would unload, load and make it fault, and
> > then fail the load before restoring it by loading again), so I went with
> > bpf_testmod.
> >
> > Maybe we can hardcode a list of tests to be executed serially in --workers=n > 1
> > mode? All serial tests are then executed in the beginning (or end), and then it
> > starts invoking others in parallel as usual.
>
> you can mark test as serial with "serial_" prefix, grep for that, we

Thanks for pointing that out!

> have a bunch of tests like this. But if you are going to unload and
> reload bpf_testmod, you will be forcing any bpf_testmod-using test to
> be serial, which I'm not sure is such a great idea.
>

Didn't get the last part, based on my reading it will execute serial tests one
by one (after finishing parallel tests), so if my serial test restores the
loaded bpf_testmod after completing, it shouldn't really impact other tests,
right? Did I miss something?

--
Kartikeya