scsi: Fix use-after-free

commit bcd8f2e9 upstream.

This patch fixes one use-after-free report[1] by KASAN.

In __scsi_scan_target(), when a type 31 device is probed,
SCSI_SCAN_TARGET_PRESENT is returned and the target will be scanned
again.

Inside the following scsi_report_lun_scan(), one new scsi_device
instance is allocated, and scsi_probe_and_add_lun() is called again to
probe the target and still see type 31 device, finally
__scsi_remove_device() is called to remove & free the device at the end
of scsi_probe_and_add_lun(), so cause use-after-free in
scsi_report_lun_scan().

And the following SCSI log can be observed:

	scsi 0:0:2:0: scsi scan: INQUIRY pass 1 length 36
	scsi 0:0:2:0: scsi scan: INQUIRY successful with code 0x0
	scsi 0:0:2:0: scsi scan: peripheral device type of 31, no device added
	scsi 0:0:2:0: scsi scan: Sending REPORT LUNS to (try 0)
	scsi 0:0:2:0: scsi scan: REPORT LUNS successful (try 0) result 0x0
	scsi 0:0:2:0: scsi scan: REPORT LUN scan
	scsi 0:0:2:0: scsi scan: INQUIRY pass 1 length 36
	scsi 0:0:2:0: scsi scan: INQUIRY successful with code 0x0
	scsi 0:0:2:0: scsi scan: peripheral device type of 31, no device added
	BUG: KASAN: use-after-free in __scsi_scan_target+0xbf8/0xe40 at addr ffff88007b44a104

This patch fixes the issue by moving the putting reference at
the end of scsi_report_lun_scan().

[1] KASAN report
==================================================================
[    3.274597] PM: Adding info for serio:serio1
[    3.275127] BUG: KASAN: use-after-free in __scsi_scan_target+0xd87/0xdf0 at addr ffff880254d8c304
[    3.275653] Read of size 4 by task kworker/u10:0/27
[    3.275903] CPU: 3 PID: 27 Comm: kworker/u10:0 Not tainted 4.8.0 #2121
[    3.276258] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[    3.276797] Workqueue: events_unbound async_run_entry_fn
[    3.277083]  ffff880254d8c380 ffff880259a37870 ffffffff94bbc6c1 ffff880078402d80
[    3.277532]  ffff880254d8bb80 ffff880259a37898 ffffffff9459fec1 ffff880259a37930
[    3.277989]  ffff880254d8bb80 ffff880078402d80 ffff880259a37920 ffffffff945a0165
[    3.278436] Call Trace:
[    3.278528]  [<ffffffff94bbc6c1>] dump_stack+0x65/0x84
[    3.278797]  [<ffffffff9459fec1>] kasan_object_err+0x21/0x70
[    3.279063] device: 'psaux': device_add
[    3.279616]  [<ffffffff945a0165>] kasan_report_error+0x205/0x500
[    3.279651] PM: Adding info for No Bus:psaux
[    3.280202]  [<ffffffff944ecd22>] ? kfree_const+0x22/0x30
[    3.280486]  [<ffffffff94bc2dc9>] ? kobject_release+0x119/0x370
[    3.280805]  [<ffffffff945a0543>] __asan_report_load4_noabort+0x43/0x50
[    3.281170]  [<ffffffff9507e1f7>] ? __scsi_scan_target+0xd87/0xdf0
[    3.281506]  [<ffffffff9507e1f7>] __scsi_scan_target+0xd87/0xdf0
[    3.281848]  [<ffffffff9507d470>] ? scsi_add_device+0x30/0x30
[    3.282156]  [<ffffffff94f7f660>] ? pm_runtime_autosuspend_expiration+0x60/0x60
[    3.282570]  [<ffffffff956ddb07>] ? _raw_spin_lock+0x17/0x40
[    3.282880]  [<ffffffff9507e505>] scsi_scan_channel+0x105/0x160
[    3.283200]  [<ffffffff9507e8a2>] scsi_scan_host_selected+0x212/0x2f0
[    3.283563]  [<ffffffff9507eb3c>] do_scsi_scan_host+0x1bc/0x250
[    3.283882]  [<ffffffff9507efc1>] do_scan_async+0x41/0x450
[    3.284173]  [<ffffffff941c1fee>] async_run_entry_fn+0xfe/0x610
[    3.284492]  [<ffffffff941a8954>] ? pwq_dec_nr_in_flight+0x124/0x2a0
[    3.284876]  [<ffffffff941d1770>] ? preempt_count_add+0x130/0x160
[    3.285207]  [<ffffffff941a9a84>] process_one_work+0x544/0x12d0
[    3.285526]  [<ffffffff941aa8e9>] worker_thread+0xd9/0x12f0
[    3.285844]  [<ffffffff941aa810>] ? process_one_work+0x12d0/0x12d0
[    3.286182]  [<ffffffff941bb365>] kthread+0x1c5/0x260
[    3.286443]  [<ffffffff940855cd>] ? __switch_to+0x88d/0x1430
[    3.286745]  [<ffffffff941bb1a0>] ? kthread_worker_fn+0x5a0/0x5a0
[    3.287085]  [<ffffffff956dde9f>] ret_from_fork+0x1f/0x40
[    3.287368]  [<ffffffff941bb1a0>] ? kthread_worker_fn+0x5a0/0x5a0
[    3.287697] Object at ffff880254d8bb80, in cache kmalloc-2048 size: 2048
[    3.288064] Allocated:
[    3.288147] PID = 27
[    3.288218]  [<ffffffff940b27ab>] save_stack_trace+0x2b/0x50
[    3.288531]  [<ffffffff9459f246>] save_stack+0x46/0xd0
[    3.288806]  [<ffffffff9459f4bd>] kasan_kmalloc+0xad/0xe0
[    3.289098]  [<ffffffff9459c07e>] __kmalloc+0x13e/0x250
[    3.289378]  [<ffffffff95078e5a>] scsi_alloc_sdev+0xea/0xcf0
[    3.289701]  [<ffffffff9507de76>] __scsi_scan_target+0xa06/0xdf0
[    3.290034]  [<ffffffff9507e505>] scsi_scan_channel+0x105/0x160
[    3.290362]  [<ffffffff9507e8a2>] scsi_scan_host_selected+0x212/0x2f0
[    3.290724]  [<ffffffff9507eb3c>] do_scsi_scan_host+0x1bc/0x250
[    3.291055]  [<ffffffff9507efc1>] do_scan_async+0x41/0x450
[    3.291354]  [<ffffffff941c1fee>] async_run_entry_fn+0xfe/0x610
[    3.291695]  [<ffffffff941a9a84>] process_one_work+0x544/0x12d0
[    3.292022]  [<ffffffff941aa8e9>] worker_thread+0xd9/0x12f0
[    3.292325]  [<ffffffff941bb365>] kthread+0x1c5/0x260
[    3.292594]  [<ffffffff956dde9f>] ret_from_fork+0x1f/0x40
[    3.292886] Freed:
[    3.292945] PID = 27
[    3.293016]  [<ffffffff940b27ab>] save_stack_trace+0x2b/0x50
[    3.293327]  [<ffffffff9459f246>] save_stack+0x46/0xd0
[    3.293600]  [<ffffffff9459fa61>] kasan_slab_free+0x71/0xb0
[    3.293916]  [<ffffffff9459bac2>] kfree+0xa2/0x1f0
[    3.294168]  [<ffffffff9508158a>] scsi_device_dev_release_usercontext+0x50a/0x730
[    3.294598]  [<ffffffff941ace9a>] execute_in_process_context+0xda/0x130
[    3.294974]  [<ffffffff9508107c>] scsi_device_dev_release+0x1c/0x20
[    3.295322]  [<ffffffff94f566f6>] device_release+0x76/0x1e0
[    3.295626]  [<ffffffff94bc2db7>] kobject_release+0x107/0x370
[    3.295942]  [<ffffffff94bc29ce>] kobject_put+0x4e/0xa0
[    3.296222]  [<ffffffff94f56e17>] put_device+0x17/0x20
[    3.296497]  [<ffffffff9505201c>] scsi_device_put+0x7c/0xa0
[    3.296801]  [<ffffffff9507e1bc>] __scsi_scan_target+0xd4c/0xdf0
[    3.297132]  [<ffffffff9507e505>] scsi_scan_channel+0x105/0x160
[    3.297458]  [<ffffffff9507e8a2>] scsi_scan_host_selected+0x212/0x2f0
[    3.297829]  [<ffffffff9507eb3c>] do_scsi_scan_host+0x1bc/0x250
[    3.298156]  [<ffffffff9507efc1>] do_scan_async+0x41/0x450
[    3.298453]  [<ffffffff941c1fee>] async_run_entry_fn+0xfe/0x610
[    3.298777]  [<ffffffff941a9a84>] process_one_work+0x544/0x12d0
[    3.299105]  [<ffffffff941aa8e9>] worker_thread+0xd9/0x12f0
[    3.299408]  [<ffffffff941bb365>] kthread+0x1c5/0x260
[    3.299676]  [<ffffffff956dde9f>] ret_from_fork+0x1f/0x40
[    3.299967] Memory state around the buggy address:
[    3.300209]  ffff880254d8c200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[    3.300608]  ffff880254d8c280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[    3.300986] >ffff880254d8c300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
[    3.301408]                    ^
[    3.301550]  ffff880254d8c380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[    3.301987]  ffff880254d8c400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[    3.302396]
==================================================================

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/scsi/scsi_scan.c

@@ -1546,12 +1546,12 @@ static int scsi_report_lun_scan(struct scsi_target *starget, int bflags,
  out_err:
 	kfree(lun_data);
  out:
-	scsi_device_put(sdev);
 	if (scsi_device_created(sdev))
 		/*
 		 * the sdev we used didn't appear in the report luns scan
 		 */
 		__scsi_remove_device(sdev);
+	scsi_device_put(sdev);
 	return ret;
 }