padata: do not leak refcount in reorder_work

A recent patch that addressed a UAF introduced a reference count leak:
the parallel_data refcount is incremented unconditionally, regardless
of the return value of queue_work(). If the work item is already queued,
the incremented refcount is never decremented.

Fix this by checking the return value of queue_work() and decrementing
the refcount when necessary.

Resolves:

Unreferenced object 0xffff9d9f421e3d80 (size 192):
  comm "cryptomgr_probe", pid 157, jiffies 4294694003
  hex dump (first 32 bytes):
    80 8b cf 41 9f 9d ff ff b8 97 e0 89 ff ff ff ff  ...A............
    d0 97 e0 89 ff ff ff ff 19 00 00 00 1f 88 23 00  ..............#.
  backtrace (crc 838fb36):
    __kmalloc_cache_noprof+0x284/0x320
    padata_alloc_pd+0x20/0x1e0
    padata_alloc_shell+0x3b/0xa0
    0xffffffffc040a54d
    cryptomgr_probe+0x43/0xc0
    kthread+0xf6/0x1f0
    ret_from_fork+0x2f/0x50
    ret_from_fork_asm+0x1a/0x30

Fixes: dd7d37c ("padata: avoid UAF for reorder_work")
Cc: <[email protected]>
Signed-off-by: Dominik Grzegorzek <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

Author: Dominik Grzegorzek

kernel/padata.c

@@ -358,7 +358,8 @@ static void padata_reorder(struct parallel_data *pd)
 		 * To avoid UAF issue, add pd ref here, and put pd ref after reorder_work finish.
 		 */
 		padata_get_pd(pd);
-		queue_work(pinst->serial_wq, &pd->reorder_work);
+		if (!queue_work(pinst->serial_wq, &pd->reorder_work))
+			padata_put_pd(pd);
 	}
 }