Re: [PATCH] bpf: Fix recursion check in trampoline

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Apr 27, 2021 at 06:10:32PM -0700, Alexei Starovoitov wrote:
> On Tue, Apr 27, 2021 at 3:42 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > The recursion check in __bpf_prog_enter and __bpf_prog_exit leaves
> > some (not inlined) functions unprotected:
> >
> > In __bpf_prog_enter:
> >   - migrate_disable is called before prog->active is checked
> >
> > In __bpf_prog_exit:
> >   - migrate_enable,rcu_read_unlock_strict are called after
> >     prog->active is decreased
> >
> > When attaching trampoline to them we get panic like:
> >
> >   traps: PANIC: double fault, error_code: 0x0
> >   double fault: 0000 [#1] SMP PTI
> >   RIP: 0010:__bpf_prog_enter+0x4/0x50
> >   ...
> >   Call Trace:
> >    <IRQ>
> >    bpf_trampoline_6442466513_0+0x18/0x1000
> >    migrate_disable+0x5/0x50
> >    __bpf_prog_enter+0x9/0x50
> >    bpf_trampoline_6442466513_0+0x18/0x1000
> >    migrate_disable+0x5/0x50
> >    __bpf_prog_enter+0x9/0x50
> >    bpf_trampoline_6442466513_0+0x18/0x1000
> >    migrate_disable+0x5/0x50
> >    __bpf_prog_enter+0x9/0x50
> >    bpf_trampoline_6442466513_0+0x18/0x1000
> >    migrate_disable+0x5/0x50
> >    ...
> >
> > Making the recursion check before the rest of the calls
> > in __bpf_prog_enter and as last call in __bpf_prog_exit.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  kernel/bpf/trampoline.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index 4aa8b52adf25..301735f7e88e 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -558,12 +558,12 @@ static void notrace inc_misses_counter(struct bpf_prog *prog)
> >  u64 notrace __bpf_prog_enter(struct bpf_prog *prog)
> >         __acquires(RCU)
> >  {
> > -       rcu_read_lock();
> > -       migrate_disable();
> >         if (unlikely(__this_cpu_inc_return(*(prog->active)) != 1)) {
> >                 inc_misses_counter(prog);
> >                 return 0;
> >         }
> > +       rcu_read_lock();
> > +       migrate_disable();
> 
> That obviously doesn't work.
> After cpu_inc the task can migrate and cpu_dec
> will happen on a different cpu likely underflowing
> the counter into negative.

ugh right

> We can either mark migrate_disable as nokprobe/notrace or have bpf
> trampoline specific denylist.
> 

I was using notrace to disable that, but that would limit
other tracers.. I'll add bpf denylist

jirka