mmc: cavium: Fix use-after-free in of_platform_device_destroy

KASAN reported the following:

[   19.338655] ==================================================================
[   19.345946] BUG: KASAN: use-after-free in of_platform_device_destroy+0x88/0x100
[   19.345966] Read of size 8 at addr fffffe01aa6f1468 by task systemd-udevd/264

[   19.345983] CPU: 1 PID: 264 Comm: systemd-udevd Not tainted 4.13.0-jang+ #737
[   19.345989] Hardware name: Cavium ThunderX CN81XX board (DT)
[   19.345995] Call trace:
[   19.346013] [<fffffc800808b1b0>] dump_backtrace+0x0/0x368
[   19.346026] [<fffffc800808b6bc>] show_stack+0x24/0x30
[   19.346040] [<fffffc8008cbb944>] dump_stack+0xa4/0xc8
[   19.346057] [<fffffc80082c2870>] print_address_description+0x68/0x258
[   19.346070] [<fffffc80082c2d70>] kasan_report+0x238/0x2f8
[   19.346082] [<fffffc80082c14a8>] __asan_load8+0x88/0xb8
[   19.346098] [<fffffc8008aacee0>] of_platform_device_destroy+0x88/0x100
[   19.346131] [<fffffc8000e02fa4>] thunder_mmc_probe+0x314/0x550 [thunderx_mmc]
[   19.346147] [<fffffc800879d560>] pci_device_probe+0x158/0x1f8
[   19.346162] [<fffffc800886e53c>] driver_probe_device+0x394/0x5f8
[   19.346174] [<fffffc800886e8f4>] __driver_attach+0x154/0x158
[   19.346185] [<fffffc800886b12c>] bus_for_each_dev+0xdc/0x140
[   19.346196] [<fffffc800886d9f8>] driver_attach+0x38/0x48
[   19.346207] [<fffffc800886d148>] bus_add_driver+0x290/0x3c8
[   19.346219] [<fffffc800886fc5c>] driver_register+0xbc/0x1a0
[   19.346232] [<fffffc800879b78c>] __pci_register_driver+0xc4/0xd8
[   19.346260] [<fffffc8000e80024>] thunder_mmc_driver_init+0x24/0x10000 [thunderx_mmc]
[   19.346273] [<fffffc8008083a80>] do_one_initcall+0x98/0x1c0
[   19.346289] [<fffffc8008177b54>] do_init_module+0xe0/0x2cc
[   19.346303] [<fffffc8008175cf0>] load_module+0x3238/0x35c0
[   19.346318] [<fffffc8008176438>] SyS_finit_module+0x190/0x1a0
[   19.346329] [<fffffc80080834a0>] __sys_trace_return+0x0/0x4

This is caused by:

  platform_device_register()
   -> platform_device_unregister(to_platform_device(dev))
	freeing struct device
   -> of_node_clear_flag(dev->of_node, ...)
	writing to the freed device

The issue is solved by increasing the reference count before calling
of_platform_device_destroy() so freeing the device is postponed after
the call.

Fixes: 8fb83b142823 ("mmc: cavium: Fix probing race with regulator")
Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/cavium-thunderx.c

@@ -7,6 +7,7 @@
  *
  * Copyright (C) 2016 Cavium Inc.
  */
+#include <linux/device.h>
 #include <linux/dma-mapping.h>
 #include <linux/interrupt.h>
 #include <linux/mmc/mmc.h>
@@ -149,8 +150,11 @@ error:
 	for (i = 0; i < CAVIUM_MAX_MMC; i++) {
 		if (host->slot[i])
 			cvm_mmc_of_slot_remove(host->slot[i]);
-		if (host->slot_pdev[i])
+		if (host->slot_pdev[i]) {
+			get_device(&host->slot_pdev[i]->dev);
 			of_platform_device_destroy(&host->slot_pdev[i]->dev, NULL);
+			put_device(&host->slot_pdev[i]->dev);
+		}
 	}
 	clk_disable_unprepare(host->clk);
 	return ret;