md/raid10: fix null-ptr-deref in raid10_sync_request

init_resync() inits mempool and sets conf->have_replacemnt at the beginning
of sync, close_sync() frees the mempool when sync is completed.

After [1] recovery might be skipped and init_resync() is called but
close_sync() is not. null-ptr-deref occurs with r10bio->dev[i].repl_bio.

The following is one way to reproduce the issue.

  1) create a array, wait for resync to complete, mddev->recovery_cp is set
     to MaxSector.
  2) recovery is woken and it is skipped. conf->have_replacement is set to
     0 in init_resync(). close_sync() not called.
  3) some io errors and rdev A is set to WantReplacement.
  4) a new device is added and set to A's replacement.
  5) recovery is woken, A have replacement, but conf->have_replacemnt is
     0. r10bio->dev[i].repl_bio will not be alloced and null-ptr-deref
     occurs.

Fix it by not calling init_resync() if recovery skipped.

[1] commit 7e83ccb ("md/raid10: Allow skipping recovery when clean arrays are assembled")
Fixes: 7e83ccb ("md/raid10: Allow skipping recovery when clean arrays are assembled")
Cc: stable@vger.kernel.org
Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230222041000.3341651-3-linan666@huaweicloud.com

Author: Li Nan

drivers/md/raid10.c

@@ -3297,10 +3297,6 @@ static sector_t raid10_sync_request(struct mddev *mddev, sector_t sector_nr,
 	sector_t chunk_mask = conf->geo.chunk_mask;
 	int page_idx = 0;
 
-	if (!mempool_initialized(&conf->r10buf_pool))
-		if (init_resync(conf))
-			return 0;
-
 	/*
 	 * Allow skipping a full rebuild for incremental assembly
 	 * of a clean array, like RAID1 does.
@@ -3316,6 +3312,10 @@ static sector_t raid10_sync_request(struct mddev *mddev, sector_t sector_nr,
 		return mddev->dev_sectors - sector_nr;
 	}
 
+	if (!mempool_initialized(&conf->r10buf_pool))
+		if (init_resync(conf))
+			return 0;
+
  skipped:
 	max_sector = mddev->dev_sectors;
 	if (test_bit(MD_RECOVERY_SYNC, &mddev->recovery) ||