netdev - Re: [PATCH bpf-next] bpf, capabilities: introduce CAP

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Date:   Thu, 29 Aug 2019 10:36:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        James Morris <jmorris@...ei.org>, Jann Horn <jannh@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, kernel-team <kernel-team@...com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF

On Thu, Aug 29, 2019 at 10:23 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Aug 29, 2019 at 08:43:23AM -0700, Andy Lutomirski wrote:
> >
> > I can imagine splitting it into three capabilities:
> >
> > CAP_TRACE_KERNEL: learn which kernel functions are called when.  This
> > would allow perf profiling, for example, but not sampling of kernel
> > regs.
> >
> > CAP_TRACE_READ_KERNEL_DATA: allow the tracing, profiling, etc features
> > that can read the kernel's data.  So you get function arguments via
> > kprobe, kernel regs, and APIs that expose probe_kernel_read()
> >
> > CAP_TRACE_USER: trace unrelated user processes
> >
> > I'm not sure the code is written in a way that makes splitting
> > CAP_TRACE_KERNEL and CAP_TRACE_READ_KERNEL_DATA, and I'm not sure that
> > CAP_TRACE_KERNEL is all that useful except for plain perf record
> > without CAP_TRACE_READ_KERNEL_DATA.  What do you all think?  I suppose
> > it could also be:
> >
> > CAP_PROFILE_KERNEL: Use perf with events that aren't kprobes or
> > tracepoints.  Does not grant the ability to sample regs or the kernel
> > stack directly.
> >
> > CAP_TRACE_KERNEL: Use all of perf, ftrace, kprobe, etc.
> >
> > CAP_TRACE_USER: Use all of perf with scope limited to user mode and uprobes.
>
> imo that makes little sense from security pov, since
> such CAP_TRACE_KERNEL (ex kprobe) can trace "unrelated user process"
> just as well. Yet not letting it do cleanly via uprobe.
> Sort of like giving a spare key for back door of the house and
> saying no, you cannot have main door key.
>

Not all combinations of capabilities make total sense.  CAP_SETUID,
for example, generally lets you get all the other capabilities.
CAP_TRACE_KERNEL + CAP_TRACE_USER makes sense.  CAP_TRACE_USER by
itself makes sense.  CAP_TRACE_READ_KERNEL_DATA without
CAP_TRACE_KERNEL does not.  I don't think this is a really a problem.