Fix racy use of anon_inode_getfd() in perf_event.c

once anon_inode_getfd() is called, you can't expect *anything* about
struct file that descriptor points to - another thread might be doing
whatever it likes with descriptor table at that point.

Cc: stable <stable@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/perf_event.c

@@ -4999,8 +4999,8 @@ SYSCALL_DEFINE5(perf_event_open,
 	struct perf_event_context *ctx;
 	struct file *event_file = NULL;
 	struct file *group_file = NULL;
+	int event_fd;
 	int fput_needed = 0;
-	int fput_needed2 = 0;
 	int err;
 
 	/* for future expandability... */
@@ -5021,12 +5021,18 @@ SYSCALL_DEFINE5(perf_event_open,
 			return -EINVAL;
 	}
 
+	event_fd = get_unused_fd_flags(O_RDWR);
+	if (event_fd < 0)
+		return event_fd;
+
 	/*
 	 * Get the target context (task or percpu):
 	 */
 	ctx = find_get_context(pid, cpu);
-	if (IS_ERR(ctx))
-		return PTR_ERR(ctx);
+	if (IS_ERR(ctx)) {
+		err = PTR_ERR(ctx);
+		goto err_fd;
+	}
 
 	/*
 	 * Look up the group leader (we will attach this event to it):
@@ -5066,13 +5072,11 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (IS_ERR(event))
 		goto err_put_context;
 
-	err = anon_inode_getfd("[perf_event]", &perf_fops, event, O_RDWR);
-	if (err < 0)
-		goto err_free_put_context;
-
-	event_file = fget_light(err, &fput_needed2);
-	if (!event_file)
+	event_file = anon_inode_getfile("[perf_event]", &perf_fops, event, O_RDWR);
+	if (IS_ERR(event_file)) {
+		err = PTR_ERR(event_file);
 		goto err_free_put_context;
+	}
 
 	if (flags & PERF_FLAG_FD_OUTPUT) {
 		err = perf_event_set_output(event, group_fd);
@@ -5093,19 +5097,19 @@ SYSCALL_DEFINE5(perf_event_open,
 	list_add_tail(&event->owner_entry, &current->perf_event_list);
 	mutex_unlock(&current->perf_event_mutex);
 
-err_fput_free_put_context:
-	fput_light(event_file, fput_needed2);
+	fput_light(group_file, fput_needed);
+	fd_install(event_fd, event_file);
+	return event_fd;
 
+err_fput_free_put_context:
+	fput(event_file);
 err_free_put_context:
-	if (err < 0)
-		free_event(event);
-
+	free_event(event);
 err_put_context:
-	if (err < 0)
-		put_ctx(ctx);
-
 	fput_light(group_file, fput_needed);
-
+	put_ctx(ctx);
+err_fd:
+	put_unused_fd(event_fd);
 	return err;
 }