On Wed, Apr 24, 2024 at 8:06 AM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > 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. 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.
