[bpf-next,2/3] bpf: Reject attaching fexit to functions annotated with __noreturn

Commit Message

Yafang Shao Feb. 11, 2025, 2:33 a.m. UTC

If we attach fexit to a function annotated with __noreturn, it will
cause an issue that the bpf trampoline image will be left over even if
the bpf link has been destroyed. Take attaching do_exit() for example. The
fexit works as follows,

  bpf_trampoline
  + __bpf_tramp_enter
    + percpu_ref_get(&tr->pcref);

  + call do_exit()

  + __bpf_tramp_exit
    + percpu_ref_put(&tr->pcref);

Since do_exit() never returns, the refcnt of the trampoline image is
never decremented, preventing it from being freed. This can be verified
with as follows,

  $ bpftool link show                                   <<<< nothing output
  $ grep "bpf_trampoline_[0-9]" /proc/kallsyms
  ffffffffc04cb000 t bpf_trampoline_6442526459    [bpf] <<<< leftover

With this change, attaching fexit probes to functions like do_exit() will
be rejected.

$ ./fexit
libbpf: prog 'fexit': BPF program load failed: -EINVAL
libbpf: prog 'fexit': -- BEGIN PROG LOAD LOG --
processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
-- END PROG LOAD LOG --
libbpf: prog 'fexit': failed to load: -EINVAL
libbpf: failed to load object 'fexit_bpf'
libbpf: failed to load BPF skeleton 'fexit_bpf': -EINVAL
failed to load BPF object -22

Signed-off-by: Yafang Shao <[e-mail]>
---
 kernel/bpf/verifier.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Comments

Song Liu Feb. 11, 2025, 5:19 p.m. UTC | #1

On Mon, Feb 10, 2025 at 6:34 PM Yafang Shao <[e-mail]> wrote:
>
> If we attach fexit to a function annotated with __noreturn, it will
> cause an issue that the bpf trampoline image will be left over even if
> the bpf link has been destroyed. Take attaching do_exit() for example. The
> fexit works as follows,
>
>   bpf_trampoline
>   + __bpf_tramp_enter
>     + percpu_ref_get(&tr->pcref);
>
>   + call do_exit()
>
>   + __bpf_tramp_exit
>     + percpu_ref_put(&tr->pcref);
>
> Since do_exit() never returns, the refcnt of the trampoline image is
> never decremented, preventing it from being freed. This can be verified
> with as follows,
>
>   $ bpftool link show                                   <<<< nothing output
>   $ grep "bpf_trampoline_[0-9]" /proc/kallsyms
>   ffffffffc04cb000 t bpf_trampoline_6442526459    [bpf] <<<< leftover
>
> With this change, attaching fexit probes to functions like do_exit() will
> be rejected.
>
> $ ./fexit
> libbpf: prog 'fexit': BPF program load failed: -EINVAL
> libbpf: prog 'fexit': -- BEGIN PROG LOAD LOG --
> processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> -- END PROG LOAD LOG --
> libbpf: prog 'fexit': failed to load: -EINVAL
> libbpf: failed to load object 'fexit_bpf'
> libbpf: failed to load BPF skeleton 'fexit_bpf': -EINVAL
> failed to load BPF object -22
>
> Signed-off-by: Yafang Shao <[e-mail]>
> ---
>  kernel/bpf/verifier.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9971c03adfd5..f7224fc61e0c 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -22841,6 +22841,13 @@ BTF_ID(func, __rcu_read_unlock)
>  #endif
>  BTF_SET_END(btf_id_deny)
>
> +/* The functions annotated with __noreturn are denied. */
> +BTF_SET_START(fexit_deny)
> +#define NORETURN(fn) BTF_ID(func, fn)
> +#include <linux/noreturns.h>
> +#undef NORETURN
> +BTF_SET_END(fexit_deny)
> +
>  static bool can_be_sleepable(struct bpf_prog *prog)
>  {
>         if (prog->type == BPF_PROG_TYPE_TRACING) {
> @@ -22929,6 +22936,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>         } else if (prog->type == BPF_PROG_TYPE_TRACING &&
>                    btf_id_set_contains(&btf_id_deny, btf_id)) {
>                 return -EINVAL;
> +       } else if (prog->expected_attach_type == BPF_TRACE_FEXIT &&
> +                  btf_id_set_contains(&fexit_deny, btf_id)) {

Please add a verifier log (with verbose()) here, so that the user knows
why the program failed to attach.

Thanks,
Song

> +               return -EINVAL;
>         }
>
>         key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);
> --
> 2.43.5
>

Yafang Shao Feb. 14, 2025, 7:30 a.m. UTC | #2

On Wed, Feb 12, 2025 at 1:19 AM Song Liu <[e-mail]> wrote:
>
> On Mon, Feb 10, 2025 at 6:34 PM Yafang Shao <[e-mail]> wrote:
> >
> > If we attach fexit to a function annotated with __noreturn, it will
> > cause an issue that the bpf trampoline image will be left over even if
> > the bpf link has been destroyed. Take attaching do_exit() for example. The
> > fexit works as follows,
> >
> >   bpf_trampoline
> >   + __bpf_tramp_enter
> >     + percpu_ref_get(&tr->pcref);
> >
> >   + call do_exit()
> >
> >   + __bpf_tramp_exit
> >     + percpu_ref_put(&tr->pcref);
> >
> > Since do_exit() never returns, the refcnt of the trampoline image is
> > never decremented, preventing it from being freed. This can be verified
> > with as follows,
> >
> >   $ bpftool link show                                   <<<< nothing output
> >   $ grep "bpf_trampoline_[0-9]" /proc/kallsyms
> >   ffffffffc04cb000 t bpf_trampoline_6442526459    [bpf] <<<< leftover
> >
> > With this change, attaching fexit probes to functions like do_exit() will
> > be rejected.
> >
> > $ ./fexit
> > libbpf: prog 'fexit': BPF program load failed: -EINVAL
> > libbpf: prog 'fexit': -- BEGIN PROG LOAD LOG --
> > processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> > -- END PROG LOAD LOG --
> > libbpf: prog 'fexit': failed to load: -EINVAL
> > libbpf: failed to load object 'fexit_bpf'
> > libbpf: failed to load BPF skeleton 'fexit_bpf': -EINVAL
> > failed to load BPF object -22
> >
> > Signed-off-by: Yafang Shao <[e-mail]>
> > ---
> >  kernel/bpf/verifier.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 9971c03adfd5..f7224fc61e0c 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -22841,6 +22841,13 @@ BTF_ID(func, __rcu_read_unlock)
> >  #endif
> >  BTF_SET_END(btf_id_deny)
> >
> > +/* The functions annotated with __noreturn are denied. */
> > +BTF_SET_START(fexit_deny)
> > +#define NORETURN(fn) BTF_ID(func, fn)
> > +#include <linux/noreturns.h>
> > +#undef NORETURN
> > +BTF_SET_END(fexit_deny)
> > +
> >  static bool can_be_sleepable(struct bpf_prog *prog)
> >  {
> >         if (prog->type == BPF_PROG_TYPE_TRACING) {
> > @@ -22929,6 +22936,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> >         } else if (prog->type == BPF_PROG_TYPE_TRACING &&
> >                    btf_id_set_contains(&btf_id_deny, btf_id)) {
> >                 return -EINVAL;
> > +       } else if (prog->expected_attach_type == BPF_TRACE_FEXIT &&
> > +                  btf_id_set_contains(&fexit_deny, btf_id)) {
>
> Please add a verifier log (with verbose()) here, so that the user knows
> why the program failed to attach.

Good suggestion.  I will add it.

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9971c03adfd5..f7224fc61e0c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -22841,6 +22841,13 @@  BTF_ID(func, __rcu_read_unlock)
 #endif
 BTF_SET_END(btf_id_deny)
 
+/* The functions annotated with __noreturn are denied. */
+BTF_SET_START(fexit_deny)
+#define NORETURN(fn) BTF_ID(func, fn)
+#include <linux/noreturns.h>
+#undef NORETURN
+BTF_SET_END(fexit_deny)
+
 static bool can_be_sleepable(struct bpf_prog *prog)
 {
 	if (prog->type == BPF_PROG_TYPE_TRACING) {
@@ -22929,6 +22936,9 @@  static int check_attach_btf_id(struct bpf_verifier_env *env)
 	} else if (prog->type == BPF_PROG_TYPE_TRACING &&
 		   btf_id_set_contains(&btf_id_deny, btf_id)) {
 		return -EINVAL;
+	} else if (prog->expected_attach_type == BPF_TRACE_FEXIT &&
+		   btf_id_set_contains(&fexit_deny, btf_id)) {
+		return -EINVAL;
 	}
 
 	key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);