linux-kernel - Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

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Date:   Wed, 25 Mar 2020 15:35:28 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>, Paul Turner <pjt@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

On 23-Mär 15:12, Kees Cook wrote:
> On Mon, Mar 23, 2020 at 02:58:18PM -0700, Casey Schaufler wrote:
> > That's not too terrible, I suppose. What would you be thinking for
> > the calls that do use call_int_hook()?
> > 
> > 	rc = call_int_hook(something, something_default, goodnesses);
> > 
> > or embedded in the macro:
> > 
> > 	rc = call_int_hook(something, goodnesses);
> 
> Oh yes, good point. The hook call already knows the name, so:

I learnt this the hard way that IRC that is passed to the
call_int_hook macro is not the same as the default value for a hook

call_int_hook accomdates for a different return value when no hook is
implemented, but it does expect the default value of the hook to be 0
as it compares the return value of the callbacks to 0 instead of the
default value whereas these special cases compare it with the default
value.

For example:

  If we define the default_value of the secid_to_secctx to
  -EOPNOTSUPP, it changes the behaviour and the BPF hook, which
  returns this default value always results in a failure.

  I noticed this when I saw a bunch of messages on my VM:

    audit: error in audit_log_task_context

  which comes from audit_log_task_context and calls
  security_secid_to_secctx which ends up being always denied by BPF.

In anycase, I am still adding the default value in LSM_HOOK and using
them in the following hooks:

 getprocattr -EINVAL
 inode_getsecurity -EOPNOTSUPP
 inode_setsecurity -EOPNOTSUPP
 setprocattr -EINVAL
 task_prctl -ENOSYS
 xfrm_state_pol_flow_match 1

Will send v6 out with these changes.

- KP

> 
> #define call_int_hook(FUNC, ...) ({                        \
>         int RC = FUNC#_default;                            \
> ...
> 
> 
> -- 
> Kees Cook
> 
>