RE: [PATCH 1/1] scsi: Fix possible buffer overflows in storvsc_queuecommand

[Michael Kelley <mikelley@microsoft.com>]

From: Xiaohui Zhang <ruc_zhangxiaohui@163.com>  Sent: Tuesday, December 8, 2020 5:19 AM
> 
> From: Zhang Xiaohui <ruc_zhangxiaohui@163.com>
> 
> storvsc_queuecommand() calls memcpy() without checking
> the destination size may trigger a buffer overflower,
> which a local user could use to cause denial of service
> or the execution of arbitrary code.
> Fix it by putting the length check before calling memcpy().
> 
> Signed-off-by: Zhang Xiaohui <ruc_zhangxiaohui@163.com>
> ---
>  drivers/scsi/storvsc_drv.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index 0c65fbd41..09b60a4c0 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -1729,6 +1729,8 @@ static int storvsc_queuecommand(struct Scsi_Host *host, struct
> scsi_cmnd *scmnd)
> 
>  	vm_srb->cdb_length = scmnd->cmd_len;
> 
> +	if (vm_srb->cdb_length > STORVSC_MAX_CMD_LEN)
> +		vm_srb->cdb_length = STORVSC_MAX_CMD_LEN;
>  	memcpy(vm_srb->cdb, scmnd->cmnd, vm_srb->cdb_length);
> 
>  	sgl = (struct scatterlist *)scsi_sglist(scmnd);
> --
> 2.17.1

At first glance, this new test isn't necessary.  storvsc_queuecommand() gets
called from scsi_dispatch_cmd(), where just before the queuecommand function
is called, the cmd_len field is checked against the maximum command length
defined for the SCSI controller.  In the case of storvsc, that maximum command
length is STORVSC_MAX_CMD_LEN as set in storvsc_probe().  There's a comment
in scsi_dispatch_cmd() that covers this exact case.

You are correct that we need to make sure there's no buffer overflow.  Are
you seeing any other path where storvsc_queuecommand() could be called
without the cmd_len being checked?

Michael