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Date: Thu, 25 Apr 2024 10:50:43 +0200
From: Benjamin Tissoires <bentiss@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
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>
Subject: Re: [PATCH bpf-next v2 11/16] bpf: wq: add bpf_wq_init
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References: <20240420-bpf_wq-v2-0-6c986a5a741f@kernel.org>
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On Apr 24 2024, Alexei Starovoitov wrote:
> On Wed, Apr 24, 2024 at 8:06 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Tue, Apr 23, 2024 at 7:55 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Sat, Apr 20, 2024 at 2:10 AM Benjamin Tissoires <bentiss@kernel.org> wrote:
> > > >
> > > > We need to teach the verifier about the second argument which is declared
> > > > 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 program
> > > > 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 might
> > > >   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 = cb->prog;
> > > >
> > > >   bpf_wq_set_callback()
> > > >     cb->prog = prog;
> > > >     bpf_prog_put(prev)
> > > >     rcu_assign_ptr(cb->callback_fn,
> > > >                    callback_fn);
> > > >                                            callback = 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_return,
> > > 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 = 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.
> 
> Considering two broken-build reports already
> I've applied the following fix:
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=dc92febf7b93da5049fe177804e6b1961fcc6bd7
> 
> that addresses the build issue on !JIT and fixes this recursion problem.

Thanks a lot for fixing this on my behalf. I was slightly puzzled by the
broken-build report but really I wasn't in position to think straight
yesterday (got a pretty big muscular pain in the back and had to go to
the doctor to get painkillers)

I haven't forgoten about the double list walk in 9/16, I'll hopefully
send the fix today.

Cheers,
Benjamin