On Thu, Aug 25, 2016 at 3:32 AM, Mickaël Salaün <mic@xxxxxxxxxxx> 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? -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
