Re: [PATCH] bpf: Add deny list of btf ids check for tracing and ext programs

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Apr 28, 2021 at 12:41:34PM -0700, Andrii Nakryiko wrote:
> On Wed, Apr 28, 2021 at 9:19 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
> > and ext 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>
> > ---
> >  kernel/bpf/verifier.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2579f6fbb5c3..4ffd64eaffda 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)
> > +
> >  static int check_attach_btf_id(struct bpf_verifier_env *env)
> >  {
> >         struct bpf_prog *prog = env->prog;
> > @@ -13171,6 +13182,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> >                 ret = bpf_lsm_verify_prog(&env->log, prog);
> >                 if (ret < 0)
> >                         return ret;
> > +       } else if ((prog->type == BPF_PROG_TYPE_TRACING ||
> > +                   prog->type == BPF_PROG_TYPE_EXT) &&
> 
> BPF_PROG_TYP_EXT can only replace other BPF programs/subprograms, it
> can't replace kernel functions, so the deny list shouldn't be checked
> for them.

right, will send new version

jirka

> 
> > +                  btf_id_set_contains(&btf_id_deny, btf_id)) {
> > +               return -EINVAL;
> >         }
> >
> >         key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);
> > --
> > 2.30.2
> >
>