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From:   Stanislav Fomichev <sdf@google.com>
Date:   Mon, 17 Apr 2023 11:10:17 -0700
Message-ID: <CAKH8qBuW+T23ZvvYf4-MPc-S+ChSOARPWpTnLqTEQmF-p_3F6w@mail.gmail.com>
Subject: Re: [PATCH net-next v4 2/4] net: socket: add sockopts blacklist for
 BPF cgroup hook
To:     Christian Brauner <brauner@kernel.org>
Cc:     Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>,
        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>,
        Kuniyuki Iwashima <kuniyu@amazon.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        linux-arch@vger.kernel.org
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On Mon, Apr 17, 2023 at 7:42=E2=80=AFAM Christian Brauner <brauner@kernel.o=
rg> wrote:
>
> On Fri, Apr 14, 2023 at 06:55:39PM -0700, 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 Chris=
tian),
> > > > > > that bpf cgroup hook can cause FD leaks when used with sockopts=
 which
> > > > > > install FDs into the process fdtable.
> > > > > >
> > > > > > After some offlist discussion it was proposed to add a blacklis=
t 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=
 enabled.
> > > > > >
> > > > >
> > > > > 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 instea=
d
> >   ignore the error from the bpf program? that would still preserve
>
> That's fine by me as well.
>
> It'd be great if the net folks could tell Alex how they would want this
> handled.

Doing the bypass seems fine with me for now. If we ever decide that
fd-based optvals are worth inspecting in bpf, we can lift that bypass.

> >   the observability aspect
>
> Please see for more details
> https://lore.kernel.org/lkml/20230411-nudelsalat-spreu-3038458f25c4@braun=
er

Thanks for the context. Yeah, sockopts are being used for a lot of
interesting things :-(

> > - or maybe we can even have a per-proto bpf_getsockopt_cleanup call tha=
t
> >   gets called whenever bpf returns an error to make sure protocols have
> >   a chance to handle that condition (and free the fd)
>
> Installing an fd into an fdtable makes it visible to userspace at which
> point calling close_fd() is doable but an absolute last resort and
> generally a good indicator of misdesign. If the bpf hook wants to make
> decisions based on the file then it should receive a struct
> file, not an fd.

SG! Then let's not over-complicate it for now and do a simple bypass.