[PATCH v2] binder: fix use-after-free due to ksys_close() during fdget()

[Todd Kjos <tkjos@android.com>]

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 fd close using task_work_add()
so ksys_close() is called after returning from binder_ioctl().

Fixes: 44d8047f1d87a ("binder: use standard functions to allocate fds")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Todd Kjos <tkjos@google.com>
---
v2:
- simplified code

If possible, please add to 4.20-final

 drivers/android/binder.c | 60 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 58 insertions(+), 2 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index c525b164d39d2f..5d0233ca852c5d 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -72,6 +72,7 @@
 #include <linux/spinlock.h>
 #include <linux/ratelimit.h>
 #include <linux/syscalls.h>
+#include <linux/task_work.h>
 
 #include <uapi/linux/android/binder.h>
 
@@ -2184,6 +2185,61 @@ static bool binder_validate_fixup(struct binder_buffer *b,
 	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;
+	int fd;
+};
+
+/**
+ * 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);
+
+	ksys_close(twcb->fd);
+	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);
+	twcb->fd = fd;
+	task_work_add(current, &twcb->twork, true);
+}
+
 static void binder_transaction_buffer_release(struct binder_proc *proc,
 					      struct binder_buffer *buffer,
 					      binder_size_t *failed_at)
@@ -2323,7 +2379,7 @@ static void binder_transaction_buffer_release(struct binder_proc *proc,
 			}
 			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",
@@ -3942,7 +3998,7 @@ static int binder_apply_fd_fixups(struct binder_transaction *t)
 		} 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);
-- 
2.20.0.405.gbc1bbc6f85-goog