binder: fix use-after-free due to ksys_close() during fdget()

44d8047f1d8 ("binder: use standard functions to allocate fds")
exposed a pre-existing issue in the binder driver.

fdget() is used in ksys_ioctl() as a performance optimization.
One of the rules associated with fdget() is that ksys_close() must
not be called between the fdget() and the fdput(). There is a case
where this requirement is not met in the binder driver which results
in the reference count dropping to 0 when the device is still in
use. This can result in use-after-free or other issues.

If userpace has passed a file-descriptor for the binder driver using
a BINDER_TYPE_FDA object, then kys_close() is called on it when
handling a binder_ioctl(BC_FREE_BUFFER) command. This violates
the assumptions for using fdget().

The problem is fixed by deferring the close using task_work_add(). A
new variant of __close_fd() was created that returns a struct file
with a reference. The fput() is deferred instead of using ksys_close().

Fixes: 44d8047f1d87a ("binder: use standard functions to allocate fds")
Suggested-by: Al Viro <[e-mail]>
Signed-off-by: Todd Kjos <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/android/binder.c

···
 #include <linux/spinlock.h>
 #include <linux/ratelimit.h>
 #include <linux/syscalls.h>
+#include <linux/task_work.h>
 
 #include <uapi/linux/android/binder.h>
 
···
 	return (fixup_offset >= last_min_offset);
 }
 
+/**
+ * struct binder_task_work_cb - for deferred close
+ *
+ * @twork:                callback_head for task work
+ * @fd:                   fd to close
+ *
+ * Structure to pass task work to be handled after
+ * returning from binder_ioctl() via task_work_add().
+ */
+struct binder_task_work_cb {
+	struct callback_head twork;
+	struct file *file;
+};
+
+/**
+ * binder_do_fd_close() - close list of file descriptors
+ * @twork:	callback head for task work
+ *
+ * It is not safe to call ksys_close() during the binder_ioctl()
+ * function if there is a chance that binder's own file descriptor
+ * might be closed. This is to meet the requirements for using
+ * fdget() (see comments for __fget_light()). Therefore use
+ * task_work_add() to schedule the close operation once we have
+ * returned from binder_ioctl(). This function is a callback
+ * for that mechanism and does the actual ksys_close() on the
+ * given file descriptor.
+ */
+static void binder_do_fd_close(struct callback_head *twork)
+{
+	struct binder_task_work_cb *twcb = container_of(twork,
+			struct binder_task_work_cb, twork);
+
+	fput(twcb->file);
+	kfree(twcb);
+}
+
+/**
+ * binder_deferred_fd_close() - schedule a close for the given file-descriptor
+ * @fd:		file-descriptor to close
+ *
+ * See comments in binder_do_fd_close(). This function is used to schedule
+ * a file-descriptor to be closed after returning from binder_ioctl().
+ */
+static void binder_deferred_fd_close(int fd)
+{
+	struct binder_task_work_cb *twcb;
+
+	twcb = kzalloc(sizeof(*twcb), GFP_KERNEL);
+	if (!twcb)
+		return;
+	init_task_work(&twcb->twork, binder_do_fd_close);
+	__close_fd_get_file(fd, &twcb->file);
+	if (twcb->file)
+		task_work_add(current, &twcb->twork, true);
+	else
+		kfree(twcb);
+}
+
 static void binder_transaction_buffer_release(struct binder_proc *proc,
 					      struct binder_buffer *buffer,
 					      binder_size_t *failed_at)
···
 			}
 			fd_array = (u32 *)(parent_buffer + (uintptr_t)fda->parent_offset);
 			for (fd_index = 0; fd_index < fda->num_fds; fd_index++)
-				ksys_close(fd_array[fd_index]);
+				binder_deferred_fd_close(fd_array[fd_index]);
 		} break;
 		default:
 			pr_err("transaction release %d bad object type %x\n",
···
 		} else if (ret) {
 			u32 *fdp = (u32 *)(t->buffer->data + fixup->offset);
 
-			ksys_close(*fdp);
+			binder_deferred_fd_close(*fdp);
 		}
 		list_del(&fixup->fixup_entry);
 		kfree(fixup);

fs/file.c

···
 }
 EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
 
+/*
+ * variant of __close_fd that gets a ref on the file for later fput
+ */
+int __close_fd_get_file(unsigned int fd, struct file **res)
+{
+	struct files_struct *files = current->files;
+	struct file *file;
+	struct fdtable *fdt;
+
+	spin_lock(&files->file_lock);
+	fdt = files_fdtable(files);
+	if (fd >= fdt->max_fds)
+		goto out_unlock;
+	file = fdt->fd[fd];
+	if (!file)
+		goto out_unlock;
+	rcu_assign_pointer(fdt->fd[fd], NULL);
+	__put_unused_fd(files, fd);
+	spin_unlock(&files->file_lock);
+	get_file(file);
+	*res = file;
+	return filp_close(file, files);
+
+out_unlock:
+	spin_unlock(&files->file_lock);
+	*res = NULL;
+	return -ENOENT;
+}
+
 void do_close_on_exec(struct files_struct *files)
 {
 	unsigned i;

include/linux/fdtable.h

···
 		      unsigned int fd, struct file *file);
 extern int __close_fd(struct files_struct *files,
 		      unsigned int fd);
+extern int __close_fd_get_file(unsigned int fd, struct file **res);
 
 extern struct kmem_cache *files_cachep;