scsi: core: Fix a use-after-free

There are two .exit_cmd_priv implementations. Both implementations use
resources associated with the SCSI host. Make sure that these resources are
still available when .exit_cmd_priv is called by waiting inside
scsi_remove_host() until the tag set has been freed.

This commit fixes the following use-after-free:

==================================================================
BUG: KASAN: use-after-free in srp_exit_cmd_priv+0x27/0xd0 [ib_srp]
Read of size 8 at addr ffff888100337000 by task multipathd/16727
Call Trace:
 <TASK>
 dump_stack_lvl+0x34/0x44
 print_report.cold+0x5e/0x5db
 kasan_report+0xab/0x120
 srp_exit_cmd_priv+0x27/0xd0 [ib_srp]
 scsi_mq_exit_request+0x4d/0x70
 blk_mq_free_rqs+0x143/0x410
 __blk_mq_free_map_and_rqs+0x6e/0x100
 blk_mq_free_tag_set+0x2b/0x160
 scsi_host_dev_release+0xf3/0x1a0
 device_release+0x54/0xe0
 kobject_put+0xa5/0x120
 device_release+0x54/0xe0
 kobject_put+0xa5/0x120
 scsi_device_dev_release_usercontext+0x4c1/0x4e0
 execute_in_process_context+0x23/0x90
 device_release+0x54/0xe0
 kobject_put+0xa5/0x120
 scsi_disk_release+0x3f/0x50
 device_release+0x54/0xe0
 kobject_put+0xa5/0x120
 disk_release+0x17f/0x1b0
 device_release+0x54/0xe0
 kobject_put+0xa5/0x120
 dm_put_table_device+0xa3/0x160 [dm_mod]
 dm_put_device+0xd0/0x140 [dm_mod]
 free_priority_group+0xd8/0x110 [dm_multipath]
 free_multipath+0x94/0xe0 [dm_multipath]
 dm_table_destroy+0xa2/0x1e0 [dm_mod]
 __dm_destroy+0x196/0x350 [dm_mod]
 dev_remove+0x10c/0x160 [dm_mod]
 ctl_ioctl+0x2c2/0x590 [dm_mod]
 dm_ctl_ioctl+0x5/0x10 [dm_mod]
 __x64_sys_ioctl+0xb4/0xf0
 dm_ctl_ioctl+0x5/0x10 [dm_mod]
 __x64_sys_ioctl+0xb4/0xf0
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

Link: https://lore.kernel.org/r/20220826002635.919423-1-bvanassche@acm.org
Fixes: 65ca846a53 ("scsi: core: Introduce {init,exit}_cmd_priv()")
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Christie <michael.christie@oracle.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: John Garry <john.garry@huawei.com>
Cc: Li Zhijian <lizhijian@fujitsu.com>
Reported-by: Li Zhijian <lizhijian@fujitsu.com>
Tested-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/hosts.c

@@ -182,6 +182,15 @@ void scsi_remove_host(struct Scsi_Host *shost)
 	mutex_unlock(&shost->scan_mutex);
 	scsi_proc_host_rm(shost);
 
+	/*
+	 * New SCSI devices cannot be attached anymore because of the SCSI host
+	 * state so drop the tag set refcnt. Wait until the tag set refcnt drops
+	 * to zero because .exit_cmd_priv implementations may need the host
+	 * pointer.
+	 */
+	kref_put(&shost->tagset_refcnt, scsi_mq_free_tags);
+	wait_for_completion(&shost->tagset_freed);
+
 	spin_lock_irqsave(shost->host_lock, flags);
 	if (scsi_host_set_state(shost, SHOST_DEL))
 		BUG_ON(scsi_host_set_state(shost, SHOST_DEL_RECOVERY));
@@ -245,6 +254,9 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 	if (error)
 		goto fail;
 
+	kref_init(&shost->tagset_refcnt);
+	init_completion(&shost->tagset_freed);
+
 	/*
 	 * Increase usage count temporarily here so that calling
 	 * scsi_autopm_put_host() will trigger runtime idle if there is
@@ -317,6 +329,7 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 	pm_runtime_disable(&shost->shost_gendev);
 	pm_runtime_set_suspended(&shost->shost_gendev);
 	pm_runtime_put_noidle(&shost->shost_gendev);
+	kref_put(&shost->tagset_refcnt, scsi_mq_free_tags);
  fail:
 	return error;
 }
@@ -350,9 +363,6 @@ static void scsi_host_dev_release(struct device *dev)
 		kfree(dev_name(&shost->shost_dev));
 	}
 
-	if (shost->tag_set.tags)
-		scsi_mq_destroy_tags(shost);
-
 	kfree(shost->shost_data);
 
 	ida_free(&host_index_ida, shost->host_no);

drivers/scsi/scsi_lib.c

@@ -1983,9 +1983,13 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 	return blk_mq_alloc_tag_set(tag_set);
 }
 
-void scsi_mq_destroy_tags(struct Scsi_Host *shost)
+void scsi_mq_free_tags(struct kref *kref)
 {
+	struct Scsi_Host *shost = container_of(kref, typeof(*shost),
+					       tagset_refcnt);
+
 	blk_mq_free_tag_set(&shost->tag_set);
+	complete(&shost->tagset_freed);
 }
 
 /**

drivers/scsi/scsi_priv.h

@@ -94,7 +94,7 @@ extern void scsi_run_host_queues(struct Scsi_Host *shost);
 extern void scsi_requeue_run_queue(struct work_struct *work);
 extern void scsi_start_queue(struct scsi_device *sdev);
 extern int scsi_mq_setup_tags(struct Scsi_Host *shost);
-extern void scsi_mq_destroy_tags(struct Scsi_Host *shost);
+extern void scsi_mq_free_tags(struct kref *kref);
 extern void scsi_exit_queue(void);
 extern void scsi_evt_thread(struct work_struct *work);
 

drivers/scsi/scsi_scan.c

@@ -340,6 +340,7 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
 		kfree(sdev);
 		goto out;
 	}
+	kref_get(&sdev->host->tagset_refcnt);
 	sdev->request_queue = q;
 	q->queuedata = sdev;
 	__scsi_init_queue(sdev->host, q);

drivers/scsi/scsi_sysfs.c

@@ -1476,6 +1476,7 @@ void __scsi_remove_device(struct scsi_device *sdev)
 	mutex_unlock(&sdev->state_mutex);
 
 	blk_mq_destroy_queue(sdev->request_queue);
+	kref_put(&sdev->host->tagset_refcnt, scsi_mq_free_tags);
 	cancel_work_sync(&sdev->requeue_work);
 
 	if (sdev->host->hostt->slave_destroy)

include/scsi/scsi_host.h

@@ -557,6 +557,8 @@ struct Scsi_Host {
 	struct scsi_host_template *hostt;
 	struct scsi_transport_template *transportt;
 
+	struct kref		tagset_refcnt;
+	struct completion	tagset_freed;
 	/* Area to keep a shared tag map */
 	struct blk_mq_tag_set	tag_set;