perf: Rework free paths

Primarily make perf_event_release_kernel() into put_event(), this will
allow kernel space to create per-task inherited events, and is safer
in general.

Also, document the free_event() assumptions.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-rk9pvr6e1d0559lxstltbztc@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 40 insertions, 26 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0de199729f04..83505a35afba 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3251,7 +3251,8 @@ static void __free_event(struct perf_event *event)
 
         call_rcu(&event->rcu_head, free_event_rcu);
 }
-static void free_event(struct perf_event *event)
+
+static void _free_event(struct perf_event *event)
 {
         irq_work_sync(&event->pending);
 
@@ -3279,42 +3280,31 @@ static void free_event(struct perf_event *event)
         if (is_cgroup_event(event))
                 perf_detach_cgroup(event);
 
-
         __free_event(event);
 }
 
-int perf_event_release_kernel(struct perf_event *event)
+/*
+ * Used to free events which have a known refcount of 1, such as in error paths
+ * where the event isn't exposed yet and inherited events.
+ */
+static void free_event(struct perf_event *event)
 {
-        struct perf_event_context *ctx = event->ctx;
-
-        WARN_ON_ONCE(ctx->parent_ctx);
-        /*
-         * There are two ways this annotation is useful:
-         *
-         *  1) there is a lock recursion from perf_event_exit_task
-         *     see the comment there.
-         *
-         *  2) there is a lock-inversion with mmap_sem through
-         *     perf_event_read_group(), which takes faults while
-         *     holding ctx->mutex, however this is called after
-         *     the last filedesc died, so there is no possibility
-         *     to trigger the AB-BA case.
-         */
-        mutex_lock_nested(&ctx->mutex, SINGLE_DEPTH_NESTING);
-        perf_remove_from_context(event, true);
-        mutex_unlock(&ctx->mutex);
-
-        free_event(event);
+        if (WARN(atomic_long_cmpxchg(&event->refcount, 1, 0) != 1,
+                                "unexpected event refcount: %ld; ptr=%p\n",
+                                atomic_long_read(&event->refcount), event)) {
+                /* leak to avoid use-after-free */
+                return;
+        }
 
-        return 0;
+        _free_event(event);
 }
-EXPORT_SYMBOL_GPL(perf_event_release_kernel);
 
 /*
  * Called when the last reference to the file is gone.
  */
 static void put_event(struct perf_event *event)
 {
+        struct perf_event_context *ctx = event->ctx;
         struct task_struct *owner;
 
         if (!atomic_long_dec_and_test(&event->refcount))
@@ -3353,9 +3343,33 @@ static void put_event(struct perf_event *event)
                 put_task_struct(owner);
         }
 
-        perf_event_release_kernel(event);
+        WARN_ON_ONCE(ctx->parent_ctx);
+        /*
+         * There are two ways this annotation is useful:
+         *
+         *  1) there is a lock recursion from perf_event_exit_task
+         *     see the comment there.
+         *
+         *  2) there is a lock-inversion with mmap_sem through
+         *     perf_event_read_group(), which takes faults while
+         *     holding ctx->mutex, however this is called after
+         *     the last filedesc died, so there is no possibility
+         *     to trigger the AB-BA case.
+         */
+        mutex_lock_nested(&ctx->mutex, SINGLE_DEPTH_NESTING);
+        perf_remove_from_context(event, true);
+        mutex_unlock(&ctx->mutex);
+
+        _free_event(event);
 }
 
+int perf_event_release_kernel(struct perf_event *event)
+{
+        put_event(event);
+        return 0;
+}
+EXPORT_SYMBOL_GPL(perf_event_release_kernel);
+
 static int perf_release(struct inode *inode, struct file *file)
 {
         put_event(file->private_data);