Re: [PATCHv2] bpf: Add deny list of btf ids check for tracing programs

[Jiri Olsa <jolsa@redhat.com>]

On Thu, May 06, 2021 at 06:36:38PM -0700, Alexei Starovoitov wrote:
> On Thu, Apr 29, 2021 at 4:47 AM 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
> >    ...
> >
> > Fixing this by adding deny list of btf ids for tracing
> > programs and checking btf id during program verification.
> > Adding above functions to this list.
> >
> > Suggested-by: Alexei Starovoitov <ast@kernel.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > v2 changes:
> >   - drop check for EXT programs [Andrii]
> >
> >  kernel/bpf/verifier.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2579f6fbb5c3..42311e51ac71 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -13112,6 +13112,17 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
> >         return 0;
> >  }
> >
> > +BTF_SET_START(btf_id_deny)
> > +BTF_ID_UNUSED
> > +#ifdef CONFIG_SMP
> > +BTF_ID(func, migrate_disable)
> > +BTF_ID(func, migrate_enable)
> > +#endif
> > +#if !defined CONFIG_PREEMPT_RCU && !defined CONFIG_TINY_RCU
> > +BTF_ID(func, rcu_read_unlock_strict)
> > +#endif
> > +BTF_SET_END(btf_id_deny)
> 
> I was wondering whether it makes sense to do this on pahole side instead ?
> It can do more flexible regex matching and excluding all such functions
> from vmlinux btf without the kernel having to do a maze of #ifdef
> depending on config.
> On one side we will lose BTF info about such functions, but what do we
> need it for?
> On the other side it will be a tiny reduction in vmlinux btf :)
> Thoughts?

we just removed the ftrace filter so BTF will have 'all' functions

I think the filtering on pahole side could cause problems like
the recent one with cubictcp_state.. it's just 3 functions, but
what if they rename? this way we at least get compilation error ;-)

I'd go with all functions in BTF and restrict attachment for those
that cause problems

jirka