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

[Peter Zijlstra]

On Thu, Oct 10, 2019 at 11:13:33AM -0400, Joel Fernandes wrote:
> On Thu, Oct 10, 2019 at 10:12:51AM +0200, Peter Zijlstra wrote:
> > +static inline int perf_allow_tracepoint(struct perf_event_attr *attr)
> >  {
> > -	return sysctl_perf_event_paranoid > 1;
> > +	if (sysctl_perf_event_paranoid > -1 && !capable(CAP_SYS_ADMIN))
> > +		return -EPERM;
> > +
> 
> Here the sysctl check of > -1 also is now coupled with a CAP_SYS_ADMIN check.
> However..
> 
> > +	return security_perf_event_open(attr, PERF_SECURITY_TRACEPOINT);
> 
> >  }
> >  
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c

> > @@ -5862,14 +5859,8 @@ static int perf_mmap(struct file *file,
> >  	lock_limit >>= PAGE_SHIFT;
> >  	locked = atomic64_read(&vma->vm_mm->pinned_vm) + extra;
> >  
> > -	if (locked > lock_limit) {
> > -		if (perf_paranoid_tracepoint_raw() && !capable(CAP_IPC_LOCK)) {
> > -			ret = -EPERM;
> > -			goto unlock;
> > -		}
> > -
> > -		ret = security_perf_event_open(&event->attr,
> > -					       PERF_SECURITY_TRACEPOINT);
> > +	if (locked > lock_limit && !capable(CAP_IPC_LOCK)) {
> > +		ret = perf_allow_tracepoint(&event->attr);
> 
> In previous code, this check did not involve a check for CAP_SYS_ADMIN.
> 
> I am Ok with adding the CAP_SYS_ADMIN check as well which does make sense to
> me for tracepoint access. But it is still a change in the logic so I wanted
> to bring it up.
> 
> Let me know any other thoughts and then I'll post a new patch.

Yes, I did notice, I found it weird.

If you have CAP_IPC_LIMIT you should be able to bust mlock memory
limits, so I don't see why we should further relate that to paranoid.

The way I wrote it, we also allow to bust the limit if we have disabled
all paranoid checks. Which makes some sense I suppose.

The original commit is this:

  459ec28ab404 ("perf_counter: Allow mmap if paranoid checks are turned off")