Re: [PATCH v2] perf_event: Add support for LSM and SELinux checks

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Oct 15, 2019 at 10:30:08AM +0200, Peter Zijlstra wrote:
> On Mon, Oct 14, 2019 at 01:03:08PM -0400, Joel Fernandes (Google) wrote:
> > In current mainline, the degree of access to perf_event_open(2) system
> > call depends on the perf_event_paranoid sysctl.  This has a number of
> > limitations:
> > 
> > 1. The sysctl is only a single value. Many types of accesses are controlled
> >    based on the single value thus making the control very limited and
> >    coarse grained.
> > 2. The sysctl is global, so if the sysctl is changed, then that means
> >    all processes get access to perf_event_open(2) opening the door to
> >    security issues.
> > 
> > This patch adds LSM and SELinux access checking which will be used in
> > Android to access perf_event_open(2) for the purposes of attaching BPF
> > programs to tracepoints, perf profiling and other operations from
> > userspace. These operations are intended for production systems.
> > 
> > 5 new LSM hooks are added:
> > 1. perf_event_open: This controls access during the perf_event_open(2)
> >    syscall itself. The hook is called from all the places that the
> >    perf_event_paranoid sysctl is checked to keep it consistent with the
> >    systctl. The hook gets passed a 'type' argument which controls CPU,
> >    kernel and tracepoint accesses (in this context, CPU, kernel and
> >    tracepoint have the same semantics as the perf_event_paranoid sysctl).
> >    Additionally, I added an 'open' type which is similar to
> >    perf_event_paranoid sysctl == 3 patch carried in Android and several other
> >    distros but was rejected in mainline [1] in 2016.
> > 
> > 2. perf_event_alloc: This allocates a new security object for the event
> >    which stores the current SID within the event. It will be useful when
> >    the perf event's FD is passed through IPC to another process which may
> >    try to read the FD. Appropriate security checks will limit access.
> > 
> > 3. perf_event_free: Called when the event is closed.
> > 
> > 4. perf_event_read: Called from the read(2) and mmap(2) syscalls for the event.
> > 
> > 5. perf_event_write: Called from the ioctl(2) syscalls for the event.
> > 
> > [1] https://lwn.net/Articles/696240/
> > 
> > Since Peter had suggest LSM hooks in 2016 [1], I am adding his
> > Suggested-by tag below.
> 
> Thanks, I've queued the patch!

Thanks!

> > To use this patch, we set the perf_event_paranoid sysctl to -1 and then
> > apply selinux checking as appropriate (default deny everything, and then
> > add policy rules to give access to domains that need it). In the future
> > we can remove the perf_event_paranoid sysctl altogether.
> 
> This I'm not sure about; the sysctl is only redundant when you actually
> use a security thingy, not everyone is. I always find them things to be
> mightily unfriendly.

Right. I was just stating the above for the folks who use the security
controls.

thanks,

 - Joel