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

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Date:   Tue, 16 May 2023 21:26:58 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Beau Belgrave <beaub@...ux.microsoft.com>,
        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>,
        dthaler@...rosoft.com, brauner@...nel.org, hch@...radead.org
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached

On Tue, 16 May 2023 17:36:28 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:


> "
> The user that will generate events must have x access to the tracing directory, e.g. chmod a+x /sys/kernel/tracing
> The user that will generate events must have rw access to the tracing/user_events_data file, e.g. chmod a+rw /sys/kernel/tracing/user_events_data
> "
> So any unpriv user can create and operate user events.
> Including seeing and enabling other user's user_events with 'ls/echo/cat' in tracefs.

It can see user_events_data, but x only gives you access into the directory.
It does not get you the contents of the files within the directory. The
above only gives access to the user_events_data. Which is to create events.

I recommended using groups and not giving access to all tasks.

> 
> 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?
> 
> > and libside [2] will also help here.  
> 
> > [2] https://github.com/compudj/libside  
> 
> That's an interesting project. It doesn't do any user_events access afaict,

I'll let Beau answer the rest.

-- Steve