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

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Message-ID: <20230518011814.GA294@W11-BEAU-MD.localdomain>
Date:   Wed, 17 May 2023 18:18:14 -0700
From:   Beau Belgrave <beaub@...ux.microsoft.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        David Vernet <void@...ifault.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Thaler <dthaler@...rosoft.com>,
        Christian Brauner <brauner@...nel.org>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached

On Wed, May 17, 2023 at 05:56:34PM -0700, Alexei Starovoitov wrote:
> On Wed, May 17, 2023 at 5:19 PM Beau Belgrave <beaub@...ux.microsoft.com> 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@...ux.microsoft.com> 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.
> 

I believe those instructions are for development only. I'll get them
changed to a more secure approach. We don't want to folks leaving it
wide open.

We should tell folks to use a group and give access to the group like
Steven said earlier.

> > 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.

The delete IOCTL is different than reg/unreg. I don't see a problem with
adding a CAP_SYSADMIN check on the delete IOCTL (and other delete paths)
to prevent this. It shouldn't affect anything we are doing to add this
and it makes it so non-admins cannot delete any events if they are given
write access to the user_events_data file.

Thanks,
-Beau