RE: [PATCH] scsi: storvsc: Fix validation for unsolicited incoming packets

[Michael Kelley]

From: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx> Sent: Tuesday, October 5, 2021 4:41 AM
> 
> The validation on the length of incoming packets performed in
> storvsc_on_channel_callback() does not apply to unsolicited
> packets with ID of 0 sent by Hyper-V.  Adjust the validation
> for such unsolicited packets.
> 
> Fixes: 91b1b640b834b2 ("scsi: storvsc: Validate length of incoming packet in storvsc_on_channel_callback()")
> Reported-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
> Reviewed-by: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> ---
> Changes since RFC[1]:
>   - Merge length checks (Haiyang Zhang)
> 
> [1] https://lore.kernel.org/all/20210928163732.5908-1-parri.andrea@xxxxxxxxx/T/#u
> 
>  drivers/scsi/storvsc_drv.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/scsi/storvsc_drv.c b/drivers/scsi/storvsc_drv.c
> index ebbbc1299c625..349c1071a98d4 100644
> --- a/drivers/scsi/storvsc_drv.c
> +++ b/drivers/scsi/storvsc_drv.c
> @@ -292,6 +292,9 @@ struct vmstorage_protocol_version {
>  #define STORAGE_CHANNEL_REMOVABLE_FLAG		0x1
>  #define STORAGE_CHANNEL_EMULATED_IDE_FLAG	0x2
> 
> +/* Lower bound on the size of unsolicited packets with ID of 0 */
> +#define VSTOR_MIN_UNSOL_PKT_SIZE		48
> +

I know you have determined experimentally that Hyper-V sends
unsolicited packets with the above length, so the idea is to validate
that the guest actually gets packets at least that big.  But I wonder if
we should think about this slightly differently.

The goal is for the storvsc driver to protect itself against bad or
malicious messages from Hyper-V.  For the unsolicited messages, the
only field that this storvsc driver needs to access is the
vstor_packet->operation field.  So an alternate approach is to set
the minimum length as small as possible while ensuring that field is valid.
Then if Hyper-V later changes the size of these unsolicited packets to
some smaller size that still contains a valid "operation" field, this code
will still work.   If in a new version of the protocol Hyper-V adds fields
that this driver needs to look at, then the minimum size can be
adjusted as needed for that new protocol version.

>  struct vstor_packet {
>  	/* Requested operation type */
>  	enum vstor_packet_operation operation;
> @@ -1285,11 +1288,15 @@ static void storvsc_on_channel_callback(void *context)
>  	foreach_vmbus_pkt(desc, channel) {
>  		struct vstor_packet *packet = hv_pkt_data(desc);
>  		struct storvsc_cmd_request *request = NULL;
> +		u32 pktlen = hv_pkt_datalen(desc);
>  		u64 rqst_id = desc->trans_id;
> +		u32 minlen = rqst_id ? sizeof(struct vstor_packet) -
> +			stor_device->vmscsi_size_delta : VSTOR_MIN_UNSOL_PKT_SIZE;
> 
> -		if (hv_pkt_datalen(desc) < sizeof(struct vstor_packet) -
> -				stor_device->vmscsi_size_delta) {
> -			dev_err(&device->device, "Invalid packet len\n");
> +		if (pktlen < minlen) {
> +			dev_err(&device->device,
> +				"Invalid pkt: id=%llu, len=%u, minlen=%u\n",
> +				rqst_id, pktlen, minlen);
>  			continue;
>  		}
> 
> --
> 2.25.1

I'm good with the rest of the code.  It's just a question of whether to perhaps
"future-proof" the code by not requiring a packet size any bigger than the
driver actually needs to reference.

Michael