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Date: Sat, 27 Aug 2016 00:40:14 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Mickaël Salaün <mic@...ikod.net>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
Arnd Bergmann <arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>,
Daniel Borkmann <daniel@...earbox.net>, Daniel Mack <daniel@...que.org>,
David Drysdale <drysdale@...gle.com>, "David S . Miller" <davem@...emloft.net>,
Elena Reshetova <elena.reshetova@...el.com>, James Morris <james.l.morris@...cle.com>,
Kees Cook <keescook@...omium.org>, Paul Moore <pmoore@...hat.com>,
Sargun Dhillon <sargun@...gun.me>, "Serge E . Hallyn" <serge@...lyn.com>, Will Drewry <wad@...omium.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>,
LSM List <linux-security-module@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>
Subject: Re: [RFC v2 00/10] Landlock LSM: Unprivileged sandboxing
On Thu, Aug 25, 2016 at 3:32 AM, Mickaël Salaün <mic@...ikod.net> wrote:
> Hi,
>
> This series is a proof of concept to fill some missing part of seccomp as the
> ability to check syscall argument pointers or creating more dynamic security
> policies. The goal of this new stackable Linux Security Module (LSM) called
> Landlock is to allow any process, including unprivileged ones, to create
> powerful security sandboxes comparable to the Seatbelt/XNU Sandbox or the
> OpenBSD Pledge. This kind of sandbox help to mitigate the security impact of
> bugs or unexpected/malicious behaviors in userland applications.
>
> The first RFC [1] was focused on extending seccomp while staying at the syscall
> level. This brought a working PoC but with some (mitigated) ToCToU race
> conditions due to the seccomp ptrace hole (now fixed) and the non-atomic
> syscall argument evaluation (hence the LSM hooks).
>
>
> # Landlock LSM
>
> This second RFC is a fresh revamp of the code while keeping some working ideas.
> This series is mainly focused on LSM hooks, while keeping the possibility to
> tied them to syscalls. This new code removes all race conditions by design. It
> now use eBPF instead of a subset of cBPF (as used by seccomp-bpf). This allow
> to remove the previous stacked cBPF hack to do complex access checks thanks to
> dedicated eBPF functions. An eBPF program is still very limited (i.e. can only
> call a whitelist of functions) and can not do a denial of service (i.e. no
> loop). The other major improvement is the replacement of the previous custom
> checker groups of syscall arguments with a new dedicated eBPF map to collect
> and compare Landlock handles with system resources (e.g. files or network
> connections).
>
> The approach taken is to add the minimum amount of code while still allowing
> the userland to create quite complex access rules. A dedicated security policy
> language such as used by SELinux, AppArmor and other major LSMs is a lot of
> code and dedicated to a trusted process (i.e. root/administrator).
>
I think there might be a problem with the current design. If I add a
seccomp filter that uses RET_LANDLOCK and some landlock filters, what
happens if a second seccomp filter *also* uses RET_LANDLOCK? I think
they'll interfere with each other. It might end up being necessary to
require only one landlock seccomp layer at a time or to find a way to
stick all the filters in a layer together with the LSM callbacks or
maybe to just drop RET_LANDLOCK and let the callbacks look at the
syscall args.
BTW, what happens if an LSM hook is called outside a syscall context,
e.g. from a page fault?
>
>
> # Sandbox example with conditional access control depending on cgroup
>
> $ mkdir /sys/fs/cgroup/sandboxed
> $ ls /home
> user1
> $ LANDLOCK_CGROUPS='/sys/fs/cgroup/sandboxed' \
> LANDLOCK_ALLOWED='/bin:/lib:/usr:/tmp:/proc/self/fd/0' \
> ./sandbox /bin/sh -i
> $ ls /home
> user1
> $ echo $$ > /sys/fs/cgroup/sandboxed/cgroup.procs
> $ ls /home
> ls: cannot open directory '/home': Permission denied
>
Something occurs to me that isn't strictly relevant to landlock but
may be relevant to unprivileged cgroups: can you cause trouble by
setting up a nastily-configured cgroup and running a setuid program in
it?
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