[Devel] [PATCH rh7 v3 3/4] fs/block_dev: fix NULL ptr deref in freeze_bdev()

[Andrey Ryabinin]

freeze_bdev() called twice on the same block device without
mounted filesystem will lead to NULL-ptr deref:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000068
 IP: [<ffffffff810b1729>] up_read+0x29/0x40

 Call Trace:
  [<ffffffff8121d2b6>] drop_super+0x16/0x30
  [<ffffffff8125942b>] freeze_bdev+0x4b/0xd0
  [<ffffffff815236db>] __dm_suspend+0xeb/0x220
  [<ffffffff8152bfa0>] ? table_load+0x390/0x390
  [<ffffffff81526aca>] dm_suspend+0xda/0x100
  [<ffffffff810b171f>] ? up_read+0x1f/0x40
  [<ffffffff8152c130>] dev_suspend+0x190/0x250
  [<ffffffff8152ca17>] ctl_ioctl+0x247/0x520
  [<ffffffff8152cd03>] dm_ctl_ioctl+0x13/0x20
  [<ffffffff8122f11e>] do_vfs_ioctl+0x27e/0x550
  [<ffffffff8122f444>] SyS_ioctl+0x54/0xa0
  [<ffffffff816bda09>] system_call_fastpath+0x16/0x1b

Check get_super() result to fix that.

https://jira.sw.ru/browse/PSBM-50858

Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 fs/block_dev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 4575c62..325ee71 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -227,7 +227,8 @@ struct super_block *freeze_bdev(struct block_device *bdev)
 		 * thaw_bdev drops it.
 		 */
 		sb = get_super(bdev);
-		drop_super(sb);
+		if (sb)
+			drop_super(sb);
 		mutex_unlock(&bdev->bd_fsfreeze_mutex);
 		return sb;
 	}
-- 
2.7.3