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From:   Stanislav Fomichev <sdf@google.com>
Date:   Tue, 25 Apr 2023 11:42:58 -0700
Message-ID: <CAKH8qBusi0AWpo_iDaFkLFPUhgZy7-p6JwhimCkpYMhWnToE7g@mail.gmail.com>
Subject: Re: handling unsupported optlen in cgroup bpf getsockopt: (was [PATCH
 net-next v4 2/4] net: socket: add sockopts blacklist for BPF cgroup hook)
To:     Kui-Feng Lee <sinquersw@gmail.com>
Cc:     Martin KaFai Lau <martin.lau@linux.dev>,
        Eric Dumazet <edumazet@google.com>, davem@davemloft.net,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        daniel@iogearbox.net, Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Leon Romanovsky <leon@kernel.org>,
        David Ahern <dsahern@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Kees Cook <keescook@chromium.org>,
        Christian Brauner <brauner@kernel.org>,
        Kuniyuki Iwashima <kuniyu@amazon.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        linux-arch@vger.kernel.org,
        Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>,
        bpf <bpf@vger.kernel.org>
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On Tue, Apr 25, 2023 at 10:59=E2=80=AFAM Kui-Feng Lee <sinquersw@gmail.com>=
 wrote:
>
>
>
> On 4/18/23 09:47, Stanislav Fomichev wrote:
> > On 04/17, Martin KaFai Lau wrote:
> >> On 4/14/23 6:55 PM, Stanislav Fomichev wrote:
> >>> On 04/13, Stanislav Fomichev wrote:
> >>>> On Thu, Apr 13, 2023 at 7:38=E2=80=AFAM Aleksandr Mikhalitsyn
> >>>> <aleksandr.mikhalitsyn@canonical.com> wrote:
> >>>>>
> >>>>> On Thu, Apr 13, 2023 at 4:22=E2=80=AFPM Eric Dumazet <edumazet@goog=
le.com> wrote:
> >>>>>>
> >>>>>> On Thu, Apr 13, 2023 at 3:35=E2=80=AFPM Alexander Mikhalitsyn
> >>>>>> <aleksandr.mikhalitsyn@canonical.com> wrote:
> >>>>>>>
> >>>>>>> During work on SO_PEERPIDFD, it was discovered (thanks to Christi=
an),
> >>>>>>> that bpf cgroup hook can cause FD leaks when used with sockopts w=
hich
> >>>>>>> install FDs into the process fdtable.
> >>>>>>>
> >>>>>>> After some offlist discussion it was proposed to add a blacklist =
of
> >>>>>>
> >>>>>> We try to replace this word by either denylist or blocklist, even =
in changelogs.
> >>>>>
> >>>>> Hi Eric,
> >>>>>
> >>>>> Oh, I'm sorry about that. :( Sure.
> >>>>>
> >>>>>>
> >>>>>>> socket options those can cause troubles when BPF cgroup hook is e=
nabled.
> >>>>>>>
> >>>>>>
> >>>>>> Can we find the appropriate Fixes: tag to help stable teams ?
> >>>>>
> >>>>> Sure, I will add next time.
> >>>>>
> >>>>> Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hook=
s")
> >>>>>
> >>>>> I think it's better to add Stanislav Fomichev to CC.
> >>>>
> >>>> Can we use 'struct proto' bpf_bypass_getsockopt instead? We already
> >>>> use it for tcp zerocopy, I'm assuming it should work in this case as
> >>>> well?
> >>>
> >>> Jakub reminded me of the other things I wanted to ask here bug forgot=
:
> >>>
> >>> - setsockopt is probably not needed, right? setsockopt hook triggers
> >>>     before the kernel and shouldn't leak anything
> >>> - for getsockopt, instead of bypassing bpf completely, should we inst=
ead
> >>>     ignore the error from the bpf program? that would still preserve
> >>>     the observability aspect
> >>
> >> stealing this thread to discuss the optlen issue which may make sense =
to
> >> bypass also.
> >>
> >> There has been issue with optlen. Other than this older post related t=
o
> >> optlen > PAGE_SIZE:
> >> https://lore.kernel.org/bpf/5c8b7d59-1f28-2284-f7b9-49d946f2e982@linux=
.dev/,
> >> the recent one related to optlen that we have seen is
> >> NETLINK_LIST_MEMBERSHIPS. The userspace passed in optlen =3D=3D 0 and =
the kernel
> >> put the expected optlen (> 0) and 'return 0;' to userspace. The usersp=
ace
> >> intention is to learn the expected optlen. This makes 'ctx.optlen >
> >> max_optlen' and __cgroup_bpf_run_filter_getsockopt() ends up returning
> >> -EFAULT to the userspace even the bpf prog has not changed anything.
> >
> > (ignoring -EFAULT issue) this seems like it needs to be
> >
> >       if (optval && (ctx.optlen > max_optlen || ctx.optlen < 0)) {
> >               /* error */
> >       }
> >
> > ?
> >
> >> Does it make sense to also bypass the bpf prog when 'ctx.optlen >
> >> max_optlen' for now (and this can use a separate patch which as usual
> >> requires a bpf selftests)?
> >
> > Yeah, makes sense. Replacing this -EFAULT with WARN_ON_ONCE or somethin=
g
> > seems like the way to go. It caused too much trouble already :-(
> >
> > Should I prepare a patch or do you want to take a stab at it?
> >
> >> In the future, does it make sense to have a specific cgroup-bpf-prog (=
a
> >> specific attach type?) that only uses bpf_dynptr kfunc to access the o=
ptval
> >> such that it can enforce read-only for some optname and potentially al=
so
> >> track if bpf-prog has written a new optval? The bpf-prog can only retu=
rn 1
> >> (OK) and only allows using bpf_set_retval() instead. Likely there is s=
till
> >> holes but could be a seed of thought to continue polishing the idea.
> >
> > Ack, let's think about it.
> >
> > Maybe we should re-evaluate 'getsockopt-happens-after-the-kernel' idea
> > as well? If we can have a sleepable hook that can copy_from_user/copy_t=
o_user,
> > and we have a mostly working bpf_getsockopt (after your refactoring),
> > I don't see why we need to continue the current scheme of triggering
> > after the kernel?
>
> Since a sleepable hook would cause some restrictions, perhaps, we could
> introduce something like the promise pattern.  In our case here, BPF
> program call an async version of copy_from_user()/copy_to_user() to
> return a promise.

Having a promise might work. This is essentially what we already do
with sockets/etc with acquire/release pattern.

What are the sleepable restrictions you're hinting about? I feel like
with the sleepable bpf, we can also remove all the temporary buffer
management / extra copies which sounds like a win to me. (we have this
ugly heuristics with BPF_SOCKOPT_KERN_BUF_SIZE) The program can
allocate temporary buffers if needed..

> >>> - or maybe we can even have a per-proto bpf_getsockopt_cleanup call t=
hat
> >>>     gets called whenever bpf returns an error to make sure protocols =
have
> >>>     a chance to handle that condition (and free the fd)
> >>>
> >>
> >>