Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull perf fixes from Thomas Gleixner:
 "A set of perf related fixes:

   - fix a CR4.PCE propagation issue caused by usage of mm instead of
     active_mm and therefore propagated the wrong value.

   - perf core fixes, which plug a use-after-free issue and make the
     event inheritance on fork more robust.

   - a tooling fix for symbol handling"

* 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  perf symbols: Fix symbols__fixup_end heuristic for corner cases
  x86/perf: Clarify why x86_pmu_event_mapped() isn't racy
  x86/perf: Fix CR4.PCE propagation to use active_mm instead of mm
  perf/core: Better explain the inherit magic
  perf/core: Simplify perf_event_free_task()
  perf/core: Fix event inheritance on fork()
  perf/core: Fix use-after-free in perf_release()

1 files changed, 48 insertions, 16 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a17ed56c8ce1..ff01cba86f43 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4256,7 +4256,7 @@ int perf_event_release_kernel(struct perf_event *event)
 
         raw_spin_lock_irq(&ctx->lock);
         /*
-         * Mark this even as STATE_DEAD, there is no external reference to it
+         * Mark this event as STATE_DEAD, there is no external reference to it
          * anymore.
          *
          * Anybody acquiring event->child_mutex after the below loop _must_
@@ -10417,21 +10417,22 @@ void perf_event_free_task(struct task_struct *task)
                         continue;
 
                 mutex_lock(&ctx->mutex);
-again:
-                list_for_each_entry_safe(event, tmp, &ctx->pinned_groups,
-                                group_entry)
-                        perf_free_event(event, ctx);
+                raw_spin_lock_irq(&ctx->lock);
+                /*
+                 * Destroy the task <-> ctx relation and mark the context dead.
+                 *
+                 * This is important because even though the task hasn't been
+                 * exposed yet the context has been (through child_list).
+                 */
+                RCU_INIT_POINTER(task->perf_event_ctxp[ctxn], NULL);
+                WRITE_ONCE(ctx->task, TASK_TOMBSTONE);
+                put_task_struct(task); /* cannot be last */
+                raw_spin_unlock_irq(&ctx->lock);
 
-                list_for_each_entry_safe(event, tmp, &ctx->flexible_groups,
-                                group_entry)
+                list_for_each_entry_safe(event, tmp, &ctx->event_list, event_entry)
                         perf_free_event(event, ctx);
 
-                if (!list_empty(&ctx->pinned_groups) ||
-                                !list_empty(&ctx->flexible_groups))
-                        goto again;
-
                 mutex_unlock(&ctx->mutex);
-
                 put_ctx(ctx);
         }
 }
@@ -10469,7 +10470,12 @@ const struct perf_event_attr *perf_event_attrs(struct perf_event *event)
 }
 
 /*
- * inherit a event from parent task to child task:
+ * Inherit a event from parent task to child task.
+ *
+ * Returns:
+ *  - valid pointer on success
+ *  - NULL for orphaned events
+ *  - IS_ERR() on error
  */
 static struct perf_event *
 inherit_event(struct perf_event *parent_event,
@@ -10563,6 +10569,16 @@ inherit_event(struct perf_event *parent_event,
         return child_event;
 }
 
+/*
+ * Inherits an event group.
+ *
+ * This will quietly suppress orphaned events; !inherit_event() is not an error.
+ * This matches with perf_event_release_kernel() removing all child events.
+ *
+ * Returns:
+ *  - 0 on success
+ *  - <0 on error
+ */
 static int inherit_group(struct perf_event *parent_event,
               struct task_struct *parent,
               struct perf_event_context *parent_ctx,
@@ -10577,6 +10593,11 @@ static int inherit_group(struct perf_event *parent_event,
                                  child, NULL, child_ctx);
         if (IS_ERR(leader))
                 return PTR_ERR(leader);
+        /*
+         * @leader can be NULL here because of is_orphaned_event(). In this
+         * case inherit_event() will create individual events, similar to what
+         * perf_group_detach() would do anyway.
+         */
         list_for_each_entry(sub, &parent_event->sibling_list, group_entry) {
                 child_ctr = inherit_event(sub, parent, parent_ctx,
                                             child, leader, child_ctx);
@@ -10586,6 +10607,17 @@ static int inherit_group(struct perf_event *parent_event,
         return 0;
 }
 
+/*
+ * Creates the child task context and tries to inherit the event-group.
+ *
+ * Clears @inherited_all on !attr.inherited or error. Note that we'll leave
+ * inherited_all set when we 'fail' to inherit an orphaned event; this is
+ * consistent with perf_event_release_kernel() removing all child events.
+ *
+ * Returns:
+ *  - 0 on success
+ *  - <0 on error
+ */
 static int
 inherit_task_group(struct perf_event *event, struct task_struct *parent,
                    struct perf_event_context *parent_ctx,
@@ -10608,7 +10640,6 @@ inherit_task_group(struct perf_event *event, struct task_struct *parent,
                  * First allocate and initialize a context for the
                  * child.
                  */
-
                 child_ctx = alloc_perf_context(parent_ctx->pmu, child);
                 if (!child_ctx)
                         return -ENOMEM;
@@ -10670,7 +10701,7 @@ static int perf_event_init_context(struct task_struct *child, int ctxn)
                 ret = inherit_task_group(event, parent, parent_ctx,
                                          child, ctxn, &inherited_all);
                 if (ret)
-                        break;
+                        goto out_unlock;
         }
 
         /*
@@ -10686,7 +10717,7 @@ static int perf_event_init_context(struct task_struct *child, int ctxn)
                 ret = inherit_task_group(event, parent, parent_ctx,
                                          child, ctxn, &inherited_all);
                 if (ret)
-                        break;
+                        goto out_unlock;
         }
 
         raw_spin_lock_irqsave(&parent_ctx->lock, flags);
@@ -10714,6 +10745,7 @@ static int perf_event_init_context(struct task_struct *child, int ctxn)
         }
 
         raw_spin_unlock_irqrestore(&parent_ctx->lock, flags);
+out_unlock:
         mutex_unlock(&parent_ctx->mutex);
 
         perf_unpin_context(parent_ctx);