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

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Message-ID: <CACYkzJ4i9qCgBRm3_pt19Tty4eR0RTMOg66f-_Rb7N3mBvgU8w@mail.gmail.com>
Date:   Tue, 24 Nov 2020 12:04:57 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     James Morris <jmorris@...ei.org>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Mimi Zohar <zohar@...ux.ibm.com>
Subject: Re: [PATCH bpf-next 2/3] bpf: Add a BPF helper for getting the IMA
 hash of an inode

On Tue, Nov 24, 2020 at 5:02 AM Alexei Starovoitov
<alexei.starovoitov@...il.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.

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;