linux-kernel - Re: [PATCH-next] scsi: fix use-after-free problem in scsi_remove

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Date:   Wed, 1 Mar 2023 11:46:31 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Zhong Jinghua <zhongjinghua@...weicloud.com>, jejb@...ux.ibm.com,
        martin.petersen@...cle.com
Cc:     linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhongjinghua@...wei.com, yi.zhang@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH-next] scsi: fix use-after-free problem in
 scsi_remove_target

On 2/12/23 19:43, Zhong Jinghua wrote:
> T0							T1
>   sdev_store_delete
>    scsi_remove_device
>     device_remove_file
>      __scsi_remove_device
>          					__iscsi_unbind_session
>          					 scsi_remove_target
> 						  spin_lock_irqsave
>          					  list_for_each_entry
>       scsi_target_reap // starget->reaf 1 -> 0

What is "reaf"? Did you perhaps want to write "reap_ref"?

> diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
> index e7893835b99a..0ad357ff4c59 100644
> --- a/drivers/scsi/scsi_sysfs.c
> +++ b/drivers/scsi/scsi_sysfs.c
> @@ -1561,7 +1561,17 @@ void scsi_remove_target(struct device *dev)
>   		    starget->state == STARGET_CREATED_REMOVE)
>   			continue;
>   		if (starget->dev.parent == dev || &starget->dev == dev) {
> -			kref_get(&starget->reap_ref);
> +
> +			/*
> +			 * If starget->reap_ref is reduced to 0, it means
> +			 * that other processes are releasing it and
> +			 * there is no need to delete it again
> +			 */
> +			if (!kref_get_unless_zero(&starget->reap_ref)) {
> +				spin_unlock_irqrestore(shost->host_lock, flags);
> +				goto restart;
> +			}
> +
>   			if (starget->state == STARGET_CREATED)
>   				starget->state = STARGET_CREATED_REMOVE;
>   			else

The above comment should be made more clear, e.g. as follows: "If the 
reference count is already zero, skip this target. Calling 
kref_get_unless_zero() if the reference count is zero is safe because 
scsi_target_destroy() will wait until the host lock has been released 
before freeing starget."

Otherwise this patch looks fine to me.

Thanks,

Bart.