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

From: Peter Zijlstra <peterz@infradead.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org, rostedt@goodmis.org,
	primiano@google.com, rsavitski@google.com, jeffv@google.com,
	kernel-team@android.com, Alexei Starovoitov <ast@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	bpf@vger.kernel.org, Daniel Borkmann <daniel@iogearbox.net>,
	Ingo Molnar <mingo@redhat.com>, James Morris <jmorris@namei.org>,
	Jiri Olsa <jolsa@redhat.com>, Kees Cook <keescook@chromium.org>,
	linux-security-module@vger.kernel.org,
	Matthew Garrett <matthewgarrett@google.com>,
	Namhyung Kim <namhyung@kernel.org>,
	selinux@vger.kernel.org, Song Liu <songliubraving@fb.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, Yonghong Song <yhs@fb.com>
Subject: Re: [PATCH RFC] perf_event: Add support for LSM and SELinux checks
Date: Thu, 10 Oct 2019 19:09:49 +0200	[thread overview]
Message-ID: <20191010170949.GR2328@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191010151333.GE96813@google.com>

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")