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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Wed, 24 Apr 2024 08:06:05 -0700
Message-ID: <CAADnVQKfzu1F=xZxyYhiocAn1iM=8f13Ca-2Jfht2dXsXuGu9A@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 11/16] bpf: wq: add bpf_wq_init
To: Benjamin Tissoires <bentiss@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, 
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>, 
	bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@vger.kernel.org>
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On Tue, Apr 23, 2024 at 7:55=E2=80=AFPM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Sat, Apr 20, 2024 at 2:10=E2=80=AFAM Benjamin Tissoires <bentiss@kerne=
l.org> wrote:
> >
> > We need to teach the verifier about the second argument which is declar=
ed
> > as void * but which is of type KF_ARG_PTR_TO_MAP. We could have dropped
> > this extra case if we declared the second argument as struct bpf_map *,
> > but that means users will have to do extra casting to have their progra=
m
> > compile.
> >
> > We also need to duplicate the timer code for the checking if the map
> > argument is matching the provided workqueue.
> >
> > Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
> >
> > ---
> >
> > FWIW, I still have one concern with this implementation:
> > - bpf_wq_work() access ->prog without protection, but I think this migh=
t
> >   be racing with bpf_wq_set_callback(): if we have the following:
> >
> >   CPU 0                                     CPU 1
> >   bpf_wq_set_callback()
> >   bpf_start()
> >                                             bpf_wq_work():
> >                                               prog =3D cb->prog;
> >
> >   bpf_wq_set_callback()
> >     cb->prog =3D prog;
> >     bpf_prog_put(prev)
> >     rcu_assign_ptr(cb->callback_fn,
> >                    callback_fn);
> >                                            callback =3D READ_ONCE(w->cb=
callback_fn);
> >
> >   As I understand callback_fn is fine, prog might be, but we clearly
> >   have an inconstency between "prog" and "callback_fn" as they can come
> >   from 2 different bpf_wq_set_callback() calls.
> >
> >   IMO we should protect this by the async->lock, but I'm not sure if
> >   it's OK or not.
>
> I see the concern, but I think it's overkill.
> Here 'prog' is used to pass it into __bpf_prog_enter_sleepable_recur()
> to keep the standard pattern of calling into sleepable prog.
> But it won't recurse.
> We can open code migrate_disable,etc from there except this_cpu_inc_retur=
n,
> but it's an overkill.
> The passed 'prog' is irrelevant.
> If somebody tries really hard by having two progs sharing the same
> map with bpf_wq and racing to set_callback... I can see how
> prog won't match callback, but it won't make a difference.
> prog is not going trigger recursion check (unless somebody
> tries is obsessed) and not going to UAF.
> I imagine it's possible to attach somewhere in core wq callback
> invocation path with fentry, set_callback to the same prog,
> and technically it's kinda sorta recursion, but different subprogs,
> so not a safety issue.
> The code as-is is fine. imo.

After sleeping on it, I realized that the use of
__bpf_prog_enter_sleepable_recur() here is very much incorrect :(
The tests are passing only because we don't inc prog->active
when we run the prog via prog_run cmd.
Adding the following:
diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index f6aad4ed2ab2..0732dfe22204 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -1514,7 +1514,9 @@ int bpf_prog_test_run_syscall(struct bpf_prog *prog,
        }

        rcu_read_lock_trace();
+       this_cpu_inc_return(*(prog->active));
        retval =3D bpf_prog_run_pin_on_cpu(prog, ctx);
+       this_cpu_dec(*(prog->active));
        rcu_read_unlock_trace();

makes the test fail sporadically.
Or 100% fail when the kernel is booted with 1 cpu.

Could you send a quick follow up to
replace __bpf_prog_enter_sleepable_recur() with
        rcu_read_lock_trace();
        migrate_disable();
?

Or I'll do it in an hour or so.