erofs: fix use-after-free of on-stack io[]

The root cause is the race as follows:
Thread #1                              Thread #2(irq ctx)

z_erofs_runqueue()
  struct z_erofs_decompressqueue io_A[];
  submit bio A
  z_erofs_decompress_kickoff(,,1)
                                       z_erofs_decompressqueue_endio(bio A)
                                       z_erofs_decompress_kickoff(,,-1)
                                       spin_lock_irqsave()
                                       atomic_add_return()
  io_wait_event()	-> pending_bios is already 0
  [end of function]
                                       wake_up_locked(io_A[]) // crash

Referenced backtrace in kernel 5.4:

[   10.129422] Unable to handle kernel paging request at virtual address eb0454a4
[   10.364157] CPU: 0 PID: 709 Comm: getprop Tainted: G        WC O      5.4.147-ab09225 #1
[   11.556325] [<c01b33b8>] (__wake_up_common) from [<c01b3300>] (__wake_up_locked+0x40/0x48)
[   11.565487] [<c01b3300>] (__wake_up_locked) from [<c044c8d0>] (z_erofs_vle_unzip_kickoff+0x6c/0xc0)
[   11.575438] [<c044c8d0>] (z_erofs_vle_unzip_kickoff) from [<c044c854>] (z_erofs_vle_read_endio+0x16c/0x17c)
[   11.586082] [<c044c854>] (z_erofs_vle_read_endio) from [<c06a80e8>] (clone_endio+0xb4/0x1d0)
[   11.595428] [<c06a80e8>] (clone_endio) from [<c04a1280>] (blk_update_request+0x150/0x4dc)
[   11.604516] [<c04a1280>] (blk_update_request) from [<c06dea28>] (mmc_blk_cqe_complete_rq+0x144/0x15c)
[   11.614640] [<c06dea28>] (mmc_blk_cqe_complete_rq) from [<c04a5d90>] (blk_done_softirq+0xb0/0xcc)
[   11.624419] [<c04a5d90>] (blk_done_softirq) from [<c010242c>] (__do_softirq+0x184/0x56c)
[   11.633419] [<c010242c>] (__do_softirq) from [<c01051e8>] (irq_exit+0xd4/0x138)
[   11.641640] [<c01051e8>] (irq_exit) from [<c010c314>] (__handle_domain_irq+0x94/0xd0)
[   11.650381] [<c010c314>] (__handle_domain_irq) from [<c04fde70>] (gic_handle_irq+0x50/0xd4)
[   11.659641] [<c04fde70>] (gic_handle_irq) from [<c0101b70>] (__irq_svc+0x70/0xb0)

Signed-off-by: Hongyu Jin <hongyu.jin@unisoc.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220401115527.4935-1-hongyu.jin.cn@gmail.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/zdata.c

@@ -1066,12 +1066,9 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
 
 	/* wake up the caller thread for sync decompression */
 	if (sync) {
-		unsigned long flags;
-
-		spin_lock_irqsave(&io->u.wait.lock, flags);
 		if (!atomic_add_return(bios, &io->pending_bios))
-			wake_up_locked(&io->u.wait);
-		spin_unlock_irqrestore(&io->u.wait.lock, flags);
+			complete(&io->u.done);
+
 		return;
 	}
 
@@ -1217,7 +1214,7 @@ jobqueue_init(struct super_block *sb,
 	} else {
 fg_out:
 		q = fgq;
-		init_waitqueue_head(&fgq->u.wait);
+		init_completion(&fgq->u.done);
 		atomic_set(&fgq->pending_bios, 0);
 	}
 	q->sb = sb;
@@ -1419,8 +1416,7 @@ static void z_erofs_runqueue(struct super_block *sb,
 		return;
 
 	/* wait until all bios are completed */
-	io_wait_event(io[JQ_SUBMIT].u.wait,
-		      !atomic_read(&io[JQ_SUBMIT].pending_bios));
+	wait_for_completion_io(&io[JQ_SUBMIT].u.done);
 
 	/* handle synchronous decompress queue in the caller context */
 	z_erofs_decompress_queue(&io[JQ_SUBMIT], pagepool);

fs/erofs/zdata.h

@@ -97,7 +97,7 @@ struct z_erofs_decompressqueue {
 	z_erofs_next_pcluster_t head;
 
 	union {
-		wait_queue_head_t wait;
+		struct completion done;
 		struct work_struct work;
 	} u;
 };