On Tue, Apr 23, 2024 at 7:55 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Sat, Apr 20, 2024 at 2:10 AM Benjamin Tissoires <bentiss@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > ---
> >
> > 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.
