Re: [PATCH] tracing/user_events: Run BPF program if attached

[Alexei Starovoitov]

On Wed, May 17, 2023 at 5:19 PM Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, May 17, 2023 at 05:10:47PM -0700, Alexei Starovoitov wrote:
> > On Wed, May 17, 2023 at 9:50 AM Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > >
> > > > > Looks like user events were designed with intention to be unprivileged.
> > > > > When I looked at kernel/trace/trace_events_user.c I assumed root.
> > > > > I doubt other people reviewed it from security perspective.
> > > > >
> > > > > Recommending "chmod a+rw /sys/kernel/tracing/user_events_data" doesn't sound like a good idea.
> > > > >
> > > > > For example, I think the following is possible:
> > > > > fd = open("/sys/kernel/tracing/user_events_data")
> > > > > ioclt(fd, DIAG_IOCSDEL)
> > > > >   user_events_ioctl_del
> > > > >      delete_user_event(info->group, name);
> > > > >
> > > > > 'info' is different for every FD, but info->group is the same for all users/processes/fds,
> > > > > because only one global init_group is created.
> > > > > So one user can unregister other user event by knowing 'name'.
> > > > > A security hole, no?
> >
> > ...
> >
> > > Regarding deleting events, only users that are given access can delete
> > > events. They must know the event name, just like users with access to
> > > delete files must know a path (and have access to it). Since the
> > > write_index and other details are per-process, unless the user has
> > > access to either /sys/kernel/tracing/events/user_events/* or
> > > /sys/kernel/tracing/user_events_status, they do not know which names are
> > > being used.
> > >
> > > If that is not enough, we could require CAP_SYSADMIN to be able to
> > > delete events even when they have access to the file. Users can also
> > > apply SELinux policies per-file to achieve further isolation, if
> > > required.
> >
> > Whether /sys/kernel/tracing/user_events_status gets g+rw
> > or it gets a+rw (as your documentation recommends)
> > it is still a security issue.
> > The "event name" is trivial to find out by looking at the source code
> > of the target process or just "string target_binary".
>
> I guess, if they have access to the binary, etc.
> So they need both access to the binary and to the tracefs directory.
> We would not give them access like this in any normal setup other than a
> developer environment.
>
> > Restricting to cap_sysadmin is not the answer, since you want unpriv.
>
> We do not need unpriv to delete events, only to write and create events.
>
> We allow unregistering call-sites, which would still work unpriv with
> this requirement.
>
> > SElinux is not the answer either.
> > Since it's unpriv, different processes should not be able to mess with
> > user events of other processes.
>
> How is this different than uprobes if we give a user access to
> /sys/kernel/tracing/dynamic_events? Users can delete those as well. I
> don't see a difference here.

Because kprobe/uprobe are root only.
No sane person will do chmod a+rw /sys/kernel/tracing/uprobe_events.
It's just like chmod a+rw /etc/passwd

Whereas this is your recommended approach for user_events.

> In our production environments we are not giving out wide security to
> this file.

Fine by me. Keep it insecure and broken. Do not send bpf patches then.
I refuse to have bpf callable from such subsystems.
Somebody will inevitably blame bpf for the insecurity of user_events.