[PATCH v4] md: Fix the null-ptr-deref of 'mddev->private' while submitting IO

[Zhihao Cheng]

Concurrent processes md_stop and IO submitting could trigger a
null-ptr-deref of 'mddev->private':

 BUG: kernel NULL pointer dereference, address: 0000000000000070
 RIP: 0010:_wait_barrier+0x2f/0x250
 Call Trace:
  raid1_make_request+0x150/0xf50
  md_handle_request+0x104/0x530
  md_submit_bio+0x76/0x130
  submit_bio+0xdd/0x250
  submit_bio_wait+0x1f/0x40
  __blkdev_direct_IO_simple+0x1f6/0x370
  blkdev_write_iter+0x3b2/0x520
  ksys_write+0x7d/0x190

              P1
  fd = open(/dev/md0, O_RDWR)
	                      P2 (forked from P1, fd' <= fd)
  write(fd)
   submit_bio
    md_handle_request
     raid1_make_request
      raid1_write_request
                                  ioctl(fd, STOP_ARRAY)
		                   mddev_set_closing_and_sync_blockdev
				   // check passed, mddev->openers = 1,
				   // because md_open() is only called
				   // once in P1->open
				   do_md_stop
				    __md_stop
				     mddev->private = NULL

       conf = mddev->private // NULL
        wait_barrier(conf, sector) // null-ptr-deref !

It is a common problem for raid0/1/10/5, and __md_stop could be triggered
by several paths(eg. ioctl, sysfs, ->dtr). Fix it by replacing
mddev_lock() with mddev_suspend_and_lock() for all __md_stop() callers.
The caller array_state_store() is guaranteed by the check
mddev_set_closing_and_sync_blockdev(mddev, 0), we just need to remove
the check '!md_is_rdwr'. For example, someone open /dev/mdx, write
something and close /dev/mdx, it won't trigger the problem, all dirty
pages can be flushed before mddev->openers decrement.
The caller dm_table_destroy() is guaranteed being invoked with device
suspended, so raid_dtr() could keep using mddev_lock_nointr().
Besides, fail the submitting IO in md_handle_request() if the
'mddev->pers' becomes NULL.

Fetch a reproducer in https://bugzilla.kernel.org/show_bug.cgi?id=222020

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+3fe892ea5fc292e1353f@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=3fe892ea5fc292e1353f
Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
---
 v1->v2:
  1. Add 'mddev->pers != NULL' check before make_request
  2. Delete dm-raid caller(->dtr) modifications
  3. Move memalloc_noio_restore after mddev_unlock_and_resume
 v2->v3:
  1. Remove modifications in array_state_store()
  2. update commit msg
 v3->v4:
  1. Remove '!md_is_rdwr' check from array_state_store()
  2. update commit msg
 drivers/md/md.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 680b34a63cb3..d81f5bf3bffe 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -414,6 +414,20 @@ bool md_handle_request(struct mddev *mddev, struct bio *bio)
 		if (!percpu_ref_tryget_live(&mddev->active_io))
 			goto check_suspended;
 	}
+	if (!mddev->pers) {
+		/*
+		 * The __md_stop() sets 'mddev->private' to NULL during
+		 * the IO submitting, check 'mddev->pers' before the IO
+		 * being processed by specific driver to avoid the
+		 * null-ptr-deref of 'mddev-><member>'. The check is
+		 * safe because the IO has got the 'mddev->active_io'
+		 * reference, and all __md_stop() callers will wait for
+		 * the reference to be zero.
+		 */
+		bio_io_error(bio);
+		percpu_ref_put(&mddev->active_io);
+		return true;
+	}
 	if (!mddev->pers->make_request(mddev, bio)) {
 		percpu_ref_put(&mddev->active_io);
 		if (mddev_is_dm(mddev) && mddev->pers->prepare_suspend)
@@ -4670,7 +4684,7 @@ array_state_store(struct mddev *mddev, const char *buf, size_t len)
 	case readonly:
 	case inactive:
 	case read_auto:
-		if (!mddev->pers || !md_is_rdwr(mddev))
+		if (!mddev->pers)
 			break;
 		/* write sysfs will not open mddev and opener should be 0 */
 		err = mddev_set_closing_and_sync_blockdev(mddev, 0);
@@ -8299,6 +8313,7 @@ static bool md_ioctl_need_suspend(unsigned int cmd)
 	case HOT_REMOVE_DISK:
 	case SET_BITMAP_FILE:
 	case SET_ARRAY_INFO:
+	case STOP_ARRAY:
 		return true;
 	default:
 		return false;
-- 
2.52.0