netdev - Re: [RFC v2 06/10] landlock: Add LSM hooks

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Date:   Tue, 30 Aug 2016 11:56:33 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        David Drysdale <drysdale@...gle.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Alexei Starovoitov <ast@...nel.org>,
        James Morris <james.l.morris@...cle.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Network Development <netdev@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Daniel Mack <daniel@...que.org>, Arnd Bergmann <arnd@...db.de>,
        Will Drewry <wad@...omium.org>, Paul Moore <pmoore@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [RFC v2 06/10] landlock: Add LSM hooks

On Aug 25, 2016 12:34 PM, "Mickaël Salaün" <mic@...ikod.net> wrote:
>
> Add LSM hooks which can be used by userland through Landlock (eBPF)
> programs. This programs are limited to a whitelist of functions (cf.
> next commit). The eBPF program context is depicted by the struct
> landlock_data (cf. include/uapi/linux/bpf.h):
> * hook: LSM hook ID (useful when using the same program for multiple LSM
>   hooks);
> * cookie: the 16-bit value from the seccomp filter that triggered this
>   Landlock program;
> * args[6]: array of LSM hook arguments.
>
> The LSM hook arguments can contain raw values as integers or
> (unleakable) pointers. The only way to use the pointers are to pass them
> to an eBPF function according to their types (e.g. the
> bpf_landlock_cmp_fs_beneath_with_struct_file function can use a struct
> file pointer).
>
> For now, there is three hooks for file system access control:
> * file_open;
> * file_permission;
> * mmap_file.
>

What's the purpose of exposing struct cred * to userspace?  It's
primarily just an optimization to save a bit of RAM, and it's a
dubious optimization at that.  What are you using it for?  Would it
make more sense to use struct task_struct * or struct pid * instead?

Also, exposing struct cred * has a really weird side-effect: it allows
(maybe even encourages) checking for pointer equality between two
struct cred * objects.  Doing so will have erratic results.