Re: [PATCH v3 2/2] perf: Refactor permissions check into perf_check_permission()

---
v3:
* Introduce this patch to refactor the permissions checking logic to
  make it more readable (reported by Eric W. Biederman).
---
 kernel/events/core.c | 58 ++++++++++++++++++++++++--------------------
 1 file changed, 32 insertions(+), 26 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f79ee82e644a..3008b986994b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c

@@ -11912,6 +11912,37 @@ __perf_event_ctx_lock_double(struct perf_event *group_leader,
        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
  *

@@ -12152,43 +12183,18 @@ 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;
        }

--

2.32.0.93.g670b81a890-goog