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

[Peter Zijlstra]

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!

> 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.