On Tue, 27 Aug 2019 16:34:47 -0700
Andy Lutomirski <luto@xxxxxxxxxx> 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.
