[RFC PATCH bpf-next seccomp 10/12] seccomp-ebpf: Add ability to read user memory

[Alexei Starovoitov]

On Tue, May 11, 2021 at 02:14:01AM -0500, YiFei Zhu wrote:
> On Mon, May 10, 2021 at 9:04 PM Alexei Starovoitov
> <alexei.starovoitov at gmail.com> wrote:
> >
> > On Mon, May 10, 2021 at 12:22:47PM -0500, YiFei Zhu wrote:
> > >
> > > +BPF_CALL_3(bpf_probe_read_user_dumpable, void *, dst, u32, size,
> > > +        const void __user *, unsafe_ptr)
> > > +{
> > > +     int ret = -EPERM;
> > > +
> > > +     if (get_dumpable(current->mm))
> > > +             ret = copy_from_user_nofault(dst, unsafe_ptr, size);
> >
> > Could you explain a bit more how dumpable flag makes it safe for unpriv?
> > The unpriv prog is attached to the children tasks only, right?
> > and dumpable gets cleared if euid changes?
> 
> This is the "reduction to ptrace". The model here is that the eBPF
> seccomp filter is doing the equivalent of ptracing the user process
> using the privileges of the task at the time of loading the seccomp
> filter.
> 
> ptrace access control is governed by ptrace.c:__ptrace_may_access. The
> requirements are:
> * always allow thread group introspection -- assume false so we are
> more restrictive than ptrace.
> * tracer has CAP_PTRACE in the target user namespace or tracer
> r/fsu/gidid equal target resu/gid -- discuss below
> * tracer has CAP_PTRACE in the target user namespace or target is
> SUID_DUMP_USER (I realized I should probably change the condition to
> == SUID_DUMP_USER).
> * passes LSM checks (eg yama ptrace_scope) -- we expose a hook to LSM
> but it's more of a "disable all advanced seccomp-eBPF features". How
> would a better interface to LSM look like?
> 
> The dumpable check handles the "target is SUID_DUMP_USER" condition,
> in the circumstance that the loader does not have CAP_PTRACE in its
> namespace at the time of load. Why would this imply its CAP_PTRACE
> capability in target namespace? This is based on my understanding on
> how capabilities and user namespaces interact:
> For the sake of simplicity, let's first assume that loader is the same
> task as the task that attaches the filter (via prctl or seccomp
> syscall).
> * Case 1: target and loader are the same user namespace. Trivial case,
> the two operations are the same.
> * Case 2: target is loader's parent namespace. Can't happen under
> assumption. Seccomp affects itself and children only, and it is only
> possible to join a descendant user ns.
> * Case 3: target is loader's descendant namespace. Loader would have
> full CAP_PTRACE on target. We are more restrictive than ptrace.
> * Case 4: target and loader are on unrelated namespace branches. Can't
> happen under assumption. Same as case 2.
> 
> Let's break this assumption and see what happens if the loader and
> attacher are in different contexts:
> * Case 1: attacher is less capable (as a general term of "what it can
> do") than loader then all of the above applies, since the model
> concerns and checks the capabilities of the loader.
> * Case 2: attacher is more capable than loader. The attacher would
> need an fd to the prog to attach it:
>   * subcase 1: attacher inherited the fd after an exec and became more
> capable. uh... why is it trusting fds from a less capable context?
>   * subcase 2: attacher has CAP_SYS_ADMIN and gets the fd via
> BPF_PROG_GET_FD_BY_ID. uh... why is it trusting random fds and
> attaching it?
>   * subcase 3: attacher received the fd via a domain socket from a
> process which may be in a different user namespace. On my first
> thought, I thought, why is it trusting random fds from a less capable
> context? Except I just thought of an adversary could:
>     * Clone into new userns,
>     * Load filter in child, which has CAP_PTRACE in new userns
>     * Send filter to the parent which doesn't have CAP_PTRACE in its userns
>     * It's broken :(
> We'll think more about this case. One way is to check against init
> namespace, which means unpriv container runtimes won't have the
> non-dumpable override. Though, it shouldn't be affecting most of the
> use cases. Alternatively we can store which userns it was loaded from
> and reject attaching from a different userns.

Typically the verifier does all the checks at load time to avoid
run-time overhead during program execution. Then at attach time we
check that attach parameters provided at load time match exactly
to those at attach time. ifindex, attach_btf_id, etc fall into this category.
Doing something similar it should be possible to avoid
doing get_dumpable() at run-time.