perf: Refactor permissions check into perf_check_permission()

Refactor the permission check in perf_event_open() into a helper
perf_check_permission(). This makes the permission check logic more
readable (because we no longer have a negated disjunction). Add a
comment mentioning the ptrace check also checks the uid.

No functional change intended.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/r/20210705084453.2151729-2-elver@google.com

kernel/events/core.c

@@ -11917,6 +11917,37 @@ again:
 	return gctx;
 }
 
+static bool
+perf_check_permission(struct perf_event_attr *attr, struct task_struct *task)
+{
+	unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
+	bool is_capable = perfmon_capable();
+
+	if (attr->sigtrap) {
+		/*
+		 * perf_event_attr::sigtrap sends signals to the other task.
+		 * Require the current task to also have CAP_KILL.
+		 */
+		rcu_read_lock();
+		is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
+		rcu_read_unlock();
+
+		/*
+		 * If the required capabilities aren't available, checks for
+		 * ptrace permissions: upgrade to ATTACH, since sending signals
+		 * can effectively change the target task.
+		 */
+		ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
+	}
+
+	/*
+	 * Preserve ptrace permission check for backwards compatibility. The
+	 * ptrace check also includes checks that the current task and other
+	 * task have matching uids, and is therefore not done here explicitly.
+	 */
+	return is_capable || ptrace_may_access(task, ptrace_mode);
+}
+
 /**
  * sys_perf_event_open - open a performance event, associate it to a task/cpu
  *
@@ -12158,43 +12189,18 @@ SYSCALL_DEFINE5(perf_event_open,
 	}
 
 	if (task) {
-		unsigned int ptrace_mode = PTRACE_MODE_READ_REALCREDS;
-		bool is_capable;
-
 		err = down_read_interruptible(&task->signal->exec_update_lock);
 		if (err)
 			goto err_file;
 
-		is_capable = perfmon_capable();
-		if (attr.sigtrap) {
-			/*
-			 * perf_event_attr::sigtrap sends signals to the other
-			 * task. Require the current task to also have
-			 * CAP_KILL.
-			 */
-			rcu_read_lock();
-			is_capable &= ns_capable(__task_cred(task)->user_ns, CAP_KILL);
-			rcu_read_unlock();
-
-			/*
-			 * If the required capabilities aren't available, checks
-			 * for ptrace permissions: upgrade to ATTACH, since
-			 * sending signals can effectively change the target
-			 * task.
-			 */
-			ptrace_mode = PTRACE_MODE_ATTACH_REALCREDS;
-		}
-
 		/*
-		 * Preserve ptrace permission check for backwards compatibility.
-		 *
 		 * We must hold exec_update_lock across this and any potential
 		 * perf_install_in_context() call for this new event to
 		 * serialize against exec() altering our credentials (and the
 		 * perf_event_exit_task() that could imply).
 		 */
 		err = -EACCES;
-		if (!is_capable && !ptrace_may_access(task, ptrace_mode))
+		if (!perf_check_permission(&attr, task))
 			goto err_cred;
 	}