Re: [PATCH bpf-next v2 11/16] bpf: wq: add bpf_wq_init

[Alexei Starovoitov]

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.