Re: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF

On Thu, Aug 29, 2019 at 10:23 AM Alexei Starovoitov
[off-list ref] wrote:

On Thu, Aug 29, 2019 at 08:43:23AM -0700, Andy Lutomirski wrote:

quoted

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.