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

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Date:   Tue, 27 Aug 2019 20:44:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Tue, 27 Aug 2019 16:34:47 -0700
Andy Lutomirski <luto@...nel.org> wrote:

> > > CAP_TRACING does not override normal permissions on sysfs or debugfs.
> > > This means that, unless a new interface for programming kprobes and
> > > such is added, it does not directly allow use of kprobes.  
> >
> > kprobes can be created in the tracefs filesystem (which is separate from
> > debugfs, tracefs just gets automatically mounted
> > in /sys/kernel/debug/tracing when debugfs is mounted) from the
> > kprobe_events file. /sys/kernel/tracing is just the tracefs
> > directory without debugfs, and was created specifically to allow
> > tracing to be access without opening up the can of worms in debugfs.  
> 
> I think that, in principle, CAP_TRACING should allow this, but I'm not
> sure how to achieve that.  I suppose we could set up
> inode_operations.permission on tracefs, but what exactly would it do?
> Would it be just like generic_permission() except that it would look
> at CAP_TRACING instead of CAP_DAC_OVERRIDE?  That is, you can use
> tracefs if you have CAP_TRACING *or* acl access?  Or would it be:
> 
> int tracing_permission(struct inode *inode, int mask)
> {
>   if (!capable(CAP_TRACING))
>     return -EPERM;
> 
>   return generic_permission(inode, mask);
> }

Perhaps we should make a group for it?

> 
> Which would mean that you need ACL *and* CAP_TRACING, so
> administrators would change the mode to 777.  That's a bit scary.
> 
> And this still doesn't let people even *find* tracefs, since it's
> hidden in debugfs.
> 
> So maybe make CAP_TRACING override ACLs but also add /sys/fs/tracing
> and mount tracefs there, too, so that regular users can at least find
> the mountpoint.

I think you missed what I said. It's not hidden in /sys/kernel/debug.
If you enable tracefs, you have /sys/kernel/tracing created, and is
completely separate from debugfs. I only have it *also* automatically
mounted to /sys/kernel/debug/tracing for backward compatibility
reasons, as older versions of trace-cmd will only mount debugfs (as
root), and expect to find it there.

 mount -t tracefs nodev /sys/kernel/tracing

-- Steve

> 
> >
> > Should we allow CAP_TRACING access to /proc/kallsyms? as it is helpful
> > to convert perf and trace-cmd's function pointers into names. Once you
> > allow tracing of the kernel, hiding /proc/kallsyms is pretty useless.  
> 
> I think we should.