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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > 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
