scsi: target/iblock: Fix overrun in WRITE SAME emulation

commit 5676234 upstream.

WRITE SAME corrupts data on the block device behind iblock if the command
is emulated. The emulation code issues (M - 1) * N times more bios than
requested, where M is the number of 512 blocks per real block size and N is
the NUMBER OF LOGICAL BLOCKS specified in WRITE SAME command. So, for a
device with 4k blocks, 7 * N more LBAs gets written after the requested
range.

The issue happens because the number of 512 byte sectors to be written is
decreased one by one while the real bios are typically from 1 to 8 512 byte
sectors per bio.

Fixes: c66ac9d ("[SCSI] target: Add LIO target core v4.0.0-rc6")
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
[bwh: Backported to 3.16: use IBLOCK_LBA_SHIFT instead of SECTOR_SHIFT]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/target/target_core_iblock.c

@@ -490,7 +490,7 @@ iblock_execute_write_same(struct se_cmd *cmd)
 
 		/* Always in 512 byte units for Linux/Block */
 		block_lba += sg->length >> IBLOCK_LBA_SHIFT;
-		sectors -= 1;
+		sectors -= sg->length >> IBLOCK_LBA_SHIFT;
 	}
 
 	iblock_submit_bios(&list, WRITE);