[4.2.y-ckt stable] Patch "target: fix COMPARE_AND_WRITE non zero SGL offset data corruption" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    target: fix COMPARE_AND_WRITE non zero SGL offset data corruption

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 0a49477f9eb85ff93acd970c2f2447aa851bb93b Mon Sep 17 00:00:00 2001
From: Jan Engelhardt <jengelh at inai.de>
Date: Mon, 23 Nov 2015 17:46:32 +0100
Subject: target: fix COMPARE_AND_WRITE non zero SGL offset data corruption

commit d94e5a61357a04938ce14d6033b4d33a3c5fd780 upstream.

target_core_sbc's compare_and_write functionality suffers from taking
data at the wrong memory location when writing a CAW request to disk
when a SGL offset is non-zero.

This can happen with loopback and vhost-scsi fabric drivers when
SCF_PASSTHROUGH_SG_TO_MEM_NOALLOC is used to map existing user-space
SGL memory into COMPARE_AND_WRITE READ/WRITE payload buffers.

Given the following sample LIO subtopology,

% targetcli ls /loopback/
o- loopback ................................. [1 Target]
  o- naa.6001405ebb8df14a ....... [naa.60014059143ed2b3]
    o- luns ................................... [2 LUNs]
      o- lun0 ................ [iblock/ram0 (/dev/ram0)]
      o- lun1 ................ [iblock/ram1 (/dev/ram1)]
% lsscsi -g
[3:0:1:0]    disk    LIO-ORG  IBLOCK           4.0   /dev/sdc   /dev/sg3
[3:0:1:1]    disk    LIO-ORG  IBLOCK           4.0   /dev/sdd   /dev/sg4

the following bug can be observed in Linux 4.3 and 4.4~rc1:

% perl -e 'print chr$_ for 0..255,reverse 0..255' >rand
% perl -e 'print "\0" x 512' >zero
% cat rand >/dev/sdd
% sg_compare_and_write -i rand -D zero --lba 0 /dev/sdd
% sg_compare_and_write -i zero -D rand --lba 0 /dev/sdd
Miscompare reported
% hexdump -Cn 512 /dev/sdd
00000000  0f 0e 0d 0c 0b 0a 09 08  07 06 05 04 03 02 01 00
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
*
00000200

Rather than writing all-zeroes as instructed with the -D file, it
corrupts the data in the sector by splicing some of the original
bytes in. The page of the first entry of cmd->t_data_sg includes the
CDB, and sg->offset is set to a position past the CDB. I presume that
sg->offset is also the right choice to use for subsequent sglist
members.

Signed-off-by: Jan Engelhardt <jengelh at netitwork.de>
Tested-by: Douglas Gilbert <dgilbert at interlog.com>
Signed-off-by: Nicholas Bellinger <nab at linux-iscsi.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/target/target_core_sbc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/target/target_core_sbc.c b/drivers/target/target_core_sbc.c
index 4b74c2d..9444c7b 100644
--- a/drivers/target/target_core_sbc.c
+++ b/drivers/target/target_core_sbc.c
@@ -529,11 +529,11 @@ static sense_reason_t compare_and_write_callback(struct se_cmd *cmd, bool succes

 		if (block_size < PAGE_SIZE) {
 			sg_set_page(&write_sg[i], m.page, block_size,
-				    block_size);
+				    m.piter.sg->offset + block_size);
 		} else {
 			sg_miter_next(&m);
 			sg_set_page(&write_sg[i], m.page, block_size,
-				    0);
+				    m.piter.sg->offset);
 		}
 		len -= block_size;
 		i++;
--
1.9.1