[PATCH v2 4/5] scsi: Avoid using reserved length byte with VPD inquiries

From: Maciej W. Rozycki
Date: Tue Apr 20 2021 - 14:02:09 EST

As discussed in a previous workaround for a BusLogic BT-958 problem with
VPD inquiries with an allocation length of 512 bytes as requested before
commit af73623f5f10 ("[SCSI] sd: Reduce buffer size for vpd request")
are rejected outright as invalid at least by some SCSI target devices as
are any requests with a non-zero value in byte #3:

scsi host0: BusLogic BT-958
scsi 0:0:0:0: Direct-Access IBM DDYS-T18350M SA5A PQ: 0 ANSI: 3
scsi 0:0:1:0: Direct-Access SEAGATE ST336607LW 0006 PQ: 0 ANSI: 3
scsi 0:0:5:0: Direct-Access IOMEGA ZIP 100 E.08 PQ: 0 ANSI: 2
[...]
scsi0: CCB #36 Target 0: Result 2 Host Adapter Status 00 Target Status 02
scsi0: CDB 12 01 00 01 06 00
scsi0: Sense 70 00 05 00 00 00 00 18 00 00 00 00 24 00 00 C0 00 03 00 [...]
sd 0:0:0:0: scsi_vpd_inquiry(0): buf[262] => -5
scsi0: CCB #37 Target 1: Result 2 Host Adapter Status 00 Target Status 02
scsi0: CDB 12 01 00 01 06 00
scsi0: Sense 70 00 05 00 00 00 00 0A 00 00 00 00 24 00 01 C8 00 03 00 [...]
sd 0:0:1:0: scsi_vpd_inquiry(0): buf[262] => -5

(here with the buffer size tweaked to 262 so as to verify if a bit in
byte #3 of the INQUIRY command is ignored and the length of 6 assumed or
tripped over, the `BusLogic=TraceErrors' parameter and trailing sense
data zeros trimmed for brevity). Note the sense key of 0x5 denoting an
illegal request.

For the record with the buffer size of 6 requests for page 0 complete
successfully and due to page truncation `scsi_get_vpd_page' proceeds
with an attempt to get inexistent page 0x89:

sd 0:0:0:0: scsi_vpd_inquiry(0): buf[6] => 7
sd 0:0:1:0: scsi_vpd_inquiry(0): buf[6] => 13
sd 0:0:0:0: scsi_vpd_inquiry(137): buf[6] => -5
sd 0:0:1:0: scsi_vpd_inquiry(137): buf[6] => -5

Upon a further investigation it has turned out at least SCSI-2 considers
byte #3 of the INQUIRY command[1] as well as byte #2 of vital product
data pages[2] reserved and expects a value of zero there. The response
from SCSI-3 devices shown above indicates the same expectation.

Therefore it is unsafe to issue INQUIRY requests unconditionally with
the allocation length beyond 255, as they may fail with an otherwise
supported request or cause undefined behaviour with some hardware.

Now we actually never do that as all our callers of `scsi_get_vpd_page'
either hardcode the buffer size to a value between 8 and 255 or
calculate it from a structure size, of which the largest is:

struct c2_inquiry {
u8 peripheral_info; /* 0 1 */
u8 page_code; /* 1 1 */
u8 reserved1; /* 2 1 */
u8 page_len; /* 3 1 */
u8 page_id[4]; /* 4 4 */
u8 sw_version[3]; /* 8 3 */
u8 sw_date[3]; /* 11 3 */
u8 features_enabled; /* 14 1 */
u8 max_lun_supported; /* 15 1 */
u8 partitions[239]; /* 16 239 */

/* size: 255, cachelines: 2, members: 10 */
/* last cacheline: 127 bytes */
};

As from commit b3ae8780b429 ("[SCSI] Add EVPD page 0x83 and 0x80 to sysfs")
we now also have the SCSI_VPD_PG_LEN macro that reflects the limitation.

However for the sake of a possible future requirement to support VPD
pages that do have a length exceeding 255 bytes and now that the danger
of using the formerly reserved byte #3 of the INQUIRY command has been
identified execute calls to `scsi_get_vpd_page' with a request size
exceeding 255 bytes in two stages, by determining the actual length of
data to be returned first and only then issuing the intended request for
full data.

References:

[1] "Information technology - Small Computer System Interface - 2",
WORKING DRAFT, X3T9.2, Project 375D, Revision 10L, 7-SEP-93, Section
8.2.5 "INQUIRY command", pp.104-108

[2] same, Section 8.3.4 "Vital product data parameters", pp.154-159

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
Fixes: 881a256d84e6 ("[SCSI] Add VPD helper")
---
Hi,

NB the SCSI-2 working draft is the only normative reference I have access
to, downloaded many years ago and not online anymore. I have more recent
vendor documents that do indicate that bytes #3 & #2 respectively are a
part of the length field, but based on empirical evidence presented here
it is unsafe to unconditionally assume that the bytes can be set to a
non-zero value. So I think it will be safest long-term if we handle it
correctly right away now that the knowledge is fresh, as past experience
with commit af73623f5f10 ("[SCSI] sd: Reduce buffer size for vpd request")
indicates the circumstances are not always correctly understood.

No backport requested as we have no current use that would request VPD
data beyond 255 bytes.

Maciej

No changes from v1.
---
drivers/scsi/scsi.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

linux-scsi-vpd-inquiry-buffer.diff
Index: linux-macro-ide/drivers/scsi/scsi.c
===================================================================
--- linux-macro-ide.orig/drivers/scsi/scsi.c
+++ linux-macro-ide/drivers/scsi/scsi.c
@@ -348,10 +348,15 @@ int scsi_get_vpd_page(struct scsi_device

/*
* Ask for all the pages supported by this device. Determine the
- * actual data length first if so required by the host, e.g.
- * BusLogic BT-958.
+ * actual data length first if the length requested is beyond 255
+ * bytes as the high order length byte used to be reserved with
+ * older SCSI standard revisions and a non-zero value there may
+ * cause either such an INQUIRY command to be rejected by a target
+ * or undefined behaviour to occur. Also do so if so required by
+ * the host, e.g. BusLogic BT-958.
*/
- if (sdev->host->no_trailing_allocation_length) {
+ if (buf_len > SCSI_VPD_PG_LEN ||
+ sdev->host->no_trailing_allocation_length) {
result = scsi_vpd_inquiry(sdev, buf, 0, min(4, buf_len));
if (result < 4)
goto fail;
@@ -377,7 +382,8 @@ int scsi_get_vpd_page(struct scsi_device
goto fail;

found:
- if (sdev->host->no_trailing_allocation_length) {
+ if (buf_len > SCSI_VPD_PG_LEN ||
+ sdev->host->no_trailing_allocation_length) {
result = scsi_vpd_inquiry(sdev, buf, page, min(4, buf_len));
if (result < 4)
goto fail;