Patch "target: fix COMPARE_AND_WRITE non zero SGL offset data corruption" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

target: fix COMPARE_AND_WRITE non zero SGL offset data corruption

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
target-fix-compare_and_write-non-zero-sgl-offset-data-corruption.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jan Engelhardt <[email protected]>
Date: Mon, 23 Nov 2015 17:46:32 +0100
Subject: target: fix COMPARE_AND_WRITE non zero SGL offset data corruption

From: Jan Engelhardt <[email protected]>

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.

---
 drivers/target/target_core_sbc.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -502,11 +502,11 @@ static sense_reason_t compare_and_write_
 
 		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++;

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/target-fix-compare_and_write-non-zero-sgl-offset-data-corruption.patch