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

[Masami Hiramatsu <mhiramat@kernel.org>]

On Mon, 30 Sep 2019 11:31:29 -0700
Kees Cook <keescook@chromium.org> wrote:

> On Sat, Sep 28, 2019 at 07:37:27PM -0400, Steven Rostedt wrote:
> > On Wed, 28 Aug 2019 21:07:24 -0700
> > Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> > > > 
> > > > This won’t make me much more comfortable, since CAP_BPF lets it do an ever-growing set of nasty things. I’d much rather one or both of two things happen:
> > > > 
> > > > 1. Give it CAP_TRACING only. It can leak my data, but it’s rather hard for it to crash my laptop, lose data, or cause other shenanigans.
> > > > 
> > > > 2. Improve it a bit do all the privileged ops are wrapped by capset().
> > > > 
> > > > Does this make sense?  I’m a security person on occasion. I find
> > > > vulnerabilities and exploit them deliberately and I break things by
> > > > accident on a regular basis. In my considered opinion, CAP_TRACING
> > > > alone, even extended to cover part of BPF as I’ve described, is
> > > > decently safe. Getting root with just CAP_TRACING will be decently
> > > > challenging, especially if I don’t get to read things like sshd’s
> > > > memory, and improvements to mitigate even that could be added.  I
> > > > am quite confident that attacks starting with CAP_TRACING will have
> > > > clear audit signatures if auditing is on.  I am also confident that
> > > > CAP_BPF *will* allow DoS and likely privilege escalation, and this
> > > > will only get more likely as BPF gets more widely used. And, if
> > > > BPF-based auditing ever becomes a thing, writing to the audit
> > > > daemon’s maps will be a great way to cover one’s tracks.  
> > > 
> > > CAP_TRACING, as I'm proposing it, will allow full tracefs access.
> > > I think Steven and Massami prefer that as well.
> > > That includes kprobe with probe_kernel_read.
> > > That also means mini-DoS by installing kprobes everywhere or running
> > > too much ftrace.
> > 
> > I was talking with Kees at Plumbers about this, and we were talking
> > about just using simple file permissions. I started playing with some
> > patches to allow the tracefs be visible but by default it would only be
> > visible by root.
> > 
> >  rwx------
> > 
> > Then a start up script (or perhaps mount options) could change the
> > group owner, and change this to:
> > 
> >  rwxrwx---
> > 
> > Where anyone in the group assigned (say "tracing") gets full access to
> > the file system.

Does it for "all" files under tracefs?

> > 
> > The more I was playing with this, the less I see the need for
> > CAP_TRACING for ftrace and reading the format files.
> 
> Nice! Thanks for playing with this. I like it because it gives us a way
> to push policy into userspace (group membership, etc), and provides a
> clean way (hopefully) do separate "read" (kernel memory confidentiality)
> from "write" (kernel memory integrity), which wouldn't have been possible
> with a single new CAP_...

 From the confidentiality point of view, if tracefs exposes traced data,
it might include in-kernel pointer and symbols, but the user still can't
see /proc/kallsyms. This means we still have several different confidentiality
for each interface.

Anyway, adding a tracefs mount option for allowing a user group to access
event format data will be a good idea. But even though, I  think we still
need the CAP_TRACING for allowing control of intrusive tracing, like kprobes
and bpf etc. (Or, do we keep those for CAP_SYS_ADMIN??)

BTW, should we request CAP_SYS_PTRACE for ftrace uprobe interface too?
It might break any user-space program (including init) if user puts a
probe on a wrong address (e.g. non instruction boundary on x86).

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>