Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Apr 27, 2023 at 9:26 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Mon, 17 Apr 2023 15:47:36 +0000
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index f61d513..3df39a5 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -842,15 +842,21 @@ static __always_inline u64 notrace bpf_prog_start_time(void)
> >  static u64 notrace __bpf_prog_enter_recur(struct bpf_prog *prog, struct bpf_tramp_run_ctx *run_ctx)
> >       __acquires(RCU)
>
> Because __bpf_prog_enter_recur() and __bpf_prog_exit_recur() can
> legitimately nest (as you pointed out later in the thread), I think my
> original plan is the way to go.
>
>
>
> >  {
> > -     rcu_read_lock();
> > -     migrate_disable();
> > -
> > -     run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx);
> > +     int bit;
> >
> > -     if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
> > +     rcu_read_lock();
> > +     bit = test_recursion_try_acquire(_THIS_IP_, _RET_IP_);
> > +     run_ctx->recursion_bit = bit;
> > +     if (bit < 0) {
> > +             preempt_disable_notrace();
> >               bpf_prog_inc_misses_counter(prog);
> > +             preempt_enable_notrace();
> >               return 0;
> >       }
> > +
> > +     migrate_disable();
>
> Just encompass the migrate_disable/enable() with the recursion protection.
>
> That is, here add:
>
>         test_recursion_release(recursion_bit);
>
> No need to save it in the run_ctx, as you can use a local variable.
>
> As I mentioned, if it passes when checking migrate_disable() it will also
> pass when checking around migrate_enable() so the two will still be paired
> properly, even if only the migrate_enable() starts recursing.
>
>
>   bit = test_recursion_try_acquire() // OK
>   if (bit < 0)
>         return;
>   migrate_disable();
>   test_recursion_release(bit);
>
>   [..]
>
>   bit = test_recursion_try_acquire() // OK
>   migrate_enable() // traced and recurses...
>
>     bit = test_recursion_try_acquire() // fails
>     if (bit < 0)
>           return; // returns here
>     migrate_disable() // does not get called.
>
> The recursion around migrate_disable/enable() is needed because it's done
> before other checks. You can't attach the test_recursion logic to the
> __bpf_prog_enter/exit() routines, because those can legitimately recurse.
>

IIUC, the acquire/release pair works as follows,

   test_recursion_try_acquire
     [ protection area ]
   test_recursion_release

After release, there will be no protection, and thus it will fail the
tools/testing/selftests/bpf/progs/recursion.c[1] test case, because
the recursion occurs in the bpf_prog_run() itself,

  __bpf_prog_enter
     test_recursion_try_acquire
     [...]
     test_recursion_release
  // no protection after the release
  bpf_prog_run()
    bpf_prog_run() // the recursion can't be prevented.
        __bpf_prog_enter
            test_recursion_try_acquire
            [...]
            test_recursion_release
       bpf_prog_run()
           bpf_prog_run()
               __bpf_prog_enter
                  test_recursion_try_acquire
                  [...]
                  test_recursion_release
              bpf_prog_run()
              [ And so on ... ]

[1]. https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/tools/testing/selftests/bpf/progs/recursion.c#n38

-- 
Regards
Yafang