linux-kernel - Re: [PATCH v2] scsi: core: Fix shost->cmd_per_lun calculation in scsi_add_host_with

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Date:   Thu, 7 Oct 2021 22:41:46 +0100
From:   John Garry <john.garry@...wei.com>
To:     Dexuan Cui <decui@...rosoft.com>, <kys@...rosoft.com>,
        <sthemmin@...rosoft.com>, <wei.liu@...nel.org>,
        <jejb@...ux.ibm.com>, <martin.petersen@...cle.com>,
        <haiyangz@...rosoft.com>, <ming.lei@...hat.com>,
        <bvanassche@....org>, <linux-scsi@...r.kernel.org>,
        <linux-hyperv@...r.kernel.org>, <longli@...rosoft.com>,
        <mikelley@...rosoft.com>
CC:     <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>
Subject: Re: [PATCH v2] scsi: core: Fix shost->cmd_per_lun calculation in
 scsi_add_host_with_dma()

On 07/10/2021 18:49, Dexuan Cui wrote:
> After commit ea2f0f77538c, a 416-CPU VM running on Hyper-V hangs during
> boot because scsi_add_host_with_dma() sets shost->cmd_per_lun to a
> negative number (the below numbers may differ in different kernel versions):
> in drivers/scsi/storvsc_drv.c, 	storvsc_drv_init() sets
> 'max_outstanding_req_per_channel' to 352, and storvsc_probe() sets
> 'max_sub_channels' to (416 - 1) / 4 = 103 and sets scsi_driver.can_queue to
> 352 * (103 + 1) * (100 - 10) / 100 = 32947, which exceeds SHRT_MAX.

I think that you just need to mention that if can_queue exceeds 
SHRT_MAX, then there is a data truncation issue.

> 
> Use min_t(int, ...) to fix the issue.
> 
> Fixes: ea2f0f77538c ("scsi: core: Cap scsi_host cmd_per_lun at can_queue")
> Cc: stable@...r.kernel.org
> Signed-off-by: Dexuan Cui <decui@...rosoft.com>

It looks ok, I'd just like to test it a bit more.

Thanks,
John

> ---
> 
> v1 tried to fix the issue by changing the storvsc driver:
> https://lwn.net/ml/linux-kernel/BYAPR21MB1270BBC14D5F1AE69FC31A16BFB09@BYAPR21MB1270.namprd21.prod.outlook.com/
> 
> v2 directly fixes the scsi core change instead as Michael Kelley and
> John Garry suggested (refer to the above link).

To be fair, it was Michael's suggestion

> 
>   drivers/scsi/hosts.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index 3f6f14f0cafb..24b72ee4246f 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -220,7 +220,8 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
>   		goto fail;
>   	}
>   
> -	shost->cmd_per_lun = min_t(short, shost->cmd_per_lun,
> +	/* Use min_t(int, ...) in case shost->can_queue exceeds SHRT_MAX */
> +	shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
>   				   shost->can_queue);
>   
>   	error = scsi_init_sense_cache(shost);
>