[PATCH bpf-next 2/3] bpf: Add a BPF helper for getting the IMA hash of an inode

[KP Singh]

On Tue, Nov 24, 2020 at 12:04 PM KP Singh <kpsingh at chromium.org> wrote:
>
> On Tue, Nov 24, 2020 at 5:02 AM Alexei Starovoitov
> <alexei.starovoitov at gmail.com> wrote:
> >
> > On Fri, Nov 20, 2020 at 01:17:07PM +0000, KP Singh wrote:
> > > +
> > > +static bool bpf_ima_inode_hash_allowed(const struct bpf_prog *prog)
> > > +{
> > > +     return bpf_lsm_is_sleepable_hook(prog->aux->attach_btf_id);
> > > +}
> > > +
> > > +BTF_ID_LIST_SINGLE(bpf_ima_inode_hash_btf_ids, struct, inode)
> > > +
> > > +const static struct bpf_func_proto bpf_ima_inode_hash_proto = {
> > > +     .func           = bpf_ima_inode_hash,
> > > +     .gpl_only       = false,
> > > +     .ret_type       = RET_INTEGER,
> > > +     .arg1_type      = ARG_PTR_TO_BTF_ID,
> > > +     .arg1_btf_id    = &bpf_ima_inode_hash_btf_ids[0],
> > > +     .arg2_type      = ARG_PTR_TO_UNINIT_MEM,
> > > +     .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
> > > +     .allowed        = bpf_ima_inode_hash_allowed,
> > > +};
> > > +
> > >  static const struct bpf_func_proto *
> > >  bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> > >  {
> > > @@ -97,6 +121,8 @@ bpf_lsm_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> > >               return &bpf_task_storage_delete_proto;
> > >       case BPF_FUNC_bprm_opts_set:
> > >               return &bpf_bprm_opts_set_proto;
> > > +     case BPF_FUNC_ima_inode_hash:
> > > +             return &bpf_ima_inode_hash_proto;
> >
> > That's not enough for correctness.
> > Not only hook has to sleepable, but the program has to be sleepable too.
> > The patch 3 should be causing all sort of kernel warnings
> > for calling mutex from preempt disabled.
> > There it calls bpf_ima_inode_hash() from SEC("lsm/file_mprotect") program.

Okay I dug into why I did not get any warnings, I do have
CONFIG_DEBUG_ATOMIC_SLEEP
and friends enabled and I do look at dmesg and... I think you misread
the diff of my patch :)

it's indeed attaching to "lsm.s/bprm_committed_creds":

[https://lore.kernel.org/bpf/CACYkzJ7Oi8wXf=9a-e=fFHJirRbD=u47z+3+M2cRTCy_1fwtgw@mail.gmail.com/T/#m8d55bf0cdda614338cecd7154476497628612f6a]

 SEC("lsm/file_mprotect")
 int BPF_PROG(test_int_hook, struct vm_area_struct *vma,
@@ -65,8 +67,11 @@ int BPF_PROG(test_void_hook, struct linux_binprm *bprm)
  __u32 key = 0;
  __u64 *value;

- if (monitored_pid == pid)
+ if (monitored_pid == pid) {
  bprm_count++;
+ ima_hash_ret = bpf_ima_inode_hash(bprm->file->f_inode,
+  &ima_hash, sizeof(ima_hash));
+ }

  bpf_copy_from_user(args, sizeof(args), (void *)bprm->vma->vm_mm->arg_start);
  bpf_copy_from_user(args, sizeof(args), (void *)bprm->mm->arg_start);
-- 

The diff makes it look like it is attaching to "lsm/file_mprotect" but
it's actually attaching to
"lsm.s/bprm_committed_creds".

Now we can either check for prod->aux->sleepable in
bpf_ima_inode_hash_allowed or
just not expose the helper to non-sleepable hooks. I went with the
latter as this is what
we do for bpf_copy_from_user.

- KP

>
> I did actually mean to use SEC("lsm.s/bprm_committed_creds"), my bad.
>
> > "lsm/" is non-sleepable. "lsm.s/" is.
> > please enable CONFIG_DEBUG_ATOMIC_SLEEP=y in your config.
>
> Oops, yes I did notice that during recent work on the test cases.
>
> Since we need a stronger check than just warnings, I am doing
> something similar to
> what we do for bpf_copy_from_user i.e.
>
>      return prog->aux->sleepable ? &bpf_ima_inode_hash_proto : NULL;