Re: [PATCH bpf-next 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

[KP Singh <kpsingh@chromium.org>]

On 03-Mär 14:44, Andrii Nakryiko wrote:
> On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@chromium.org> wrote:
> >
> > From: KP Singh <kpsingh@google.com>
> >
> > - Functions that are whitlisted by for error injection i.e.
> >   within_error_injection_list.
> > - Security hooks, this is expected to be cleaned up after the KRSI
> >   patches introduce the LSM_HOOK macro:
> >
> >     https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/
> 
> Commit message can use a bit more work for sure. Why (and even what)
> of the changes is not really explained well.

Added some more details.

> 
> >
> > - The attachment is currently limited to functions that return an int.
> >   This can be extended later other types (e.g. PTR).
> >
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > ---
> >  kernel/bpf/btf.c      | 28 ++++++++++++++++++++--------
> >  kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
> >  2 files changed, 51 insertions(+), 8 deletions(-)
> >

[...]

> > +                       t = btf_type_skip_modifiers(btf, t->type, NULL);
> > +                       if (!btf_type_is_int(t)) {
> 
> Should the size of int be verified here? E.g., if some function
> returns u8, is that ok for BPF program to return, say, (1<<30) ?

Would this work?

       if (size != t->size) {
               bpf_log(log,
                       "size accessed = %d should be %d\n",
                       size, t->size);
               return false;
       }

- KP

> 
> > +                               bpf_log(log,
> > +                                       "ret type %s not allowed for fmod_ret\n",
> > +                                       btf_kind_str[BTF_INFO_KIND(t->info)]);
> > +                               return false;
> > +                       }
> > +               }
> >         } else if (arg >= nr_args) {
> >                 bpf_log(log, "func '%s' doesn't have %d-th argument\n",
> >                         tname, arg + 1);
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2460c8e6b5be..ae32517d4ccd 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/sort.h>
> >  #include <linux/perf_event.h>
> >  #include <linux/ctype.h>
> > +#include <linux/error-injection.h>
> >
> >  #include "disasm.h"
> >
> > @@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
> >
> >         return 0;
> >  }
> > +#define SECURITY_PREFIX "security_"
> > +
> > +static int check_attach_modify_return(struct bpf_verifier_env *env)
> > +{
> > +       struct bpf_prog *prog = env->prog;
> > +       unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
> > +
> > +       if (within_error_injection_list(addr))
> > +               return 0;
> > +
> > +       /* This is expected to be cleaned up in the future with the KRSI effort
> > +        * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
> > +        */
> > +       if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
> > +                    sizeof(SECURITY_PREFIX) - 1)) {
> > +
> > +               if (!capable(CAP_MAC_ADMIN))
> > +                       return -EPERM;
> > +
> > +               return 0;
> > +       }
> > +
> > +       verbose(env, "fmod_ret attach_btf_id %u (%s) is not modifiable\n",
> > +               prog->aux->attach_btf_id, prog->aux->attach_func_name);
> > +
> > +       return -EINVAL;
> > +}
> >
> >  static int check_attach_btf_id(struct bpf_verifier_env *env)
> >  {
> > @@ -10000,6 +10028,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> >                 }
> >                 tr->func.addr = (void *)addr;
> >                 prog->aux->trampoline = tr;
> > +
> > +               if (prog->expected_attach_type == BPF_MODIFY_RETURN)
> > +                       ret = check_attach_modify_return(env);
> >  out:
> >                 mutex_unlock(&tr->mutex);
> >                 if (ret)
> > --
> > 2.20.1
> >