scsi: qla2xxx: Fix an integer overflow in sysfs code

The value of "size" comes from the user.  When we add "start + size" it
could lead to an integer overflow bug.

It means we vmalloc() a lot more memory than we had intended.  I believe
that on 64 bit systems vmalloc() can succeed even if we ask it to
allocate huge 4GB buffers.  So we would get memory corruption and likely
a crash when we call ha->isp_ops->write_optrom() and ->read_optrom().

Only root can trigger this bug.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=194061

Cc: <stable@vger.kernel.org>
Fixes: b7cc176c9e ("[SCSI] qla2xxx: Allow region-based flash-part accesses.")
Reported-by: shqking <shqking@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/qla2xxx/qla_attr.c

@@ -318,6 +318,8 @@ qla2x00_sysfs_write_optrom_ctl(struct file *filp, struct kobject *kobj,
 		return -EINVAL;
 	if (start > ha->optrom_size)
 		return -EINVAL;
+	if (size > ha->optrom_size - start)
+		size = ha->optrom_size - start;
 
 	mutex_lock(&ha->optrom_mutex);
 	switch (val) {
@@ -343,8 +345,7 @@ qla2x00_sysfs_write_optrom_ctl(struct file *filp, struct kobject *kobj,
 		}
 
 		ha->optrom_region_start = start;
-		ha->optrom_region_size = start + size > ha->optrom_size ?
-		    ha->optrom_size - start : size;
+		ha->optrom_region_size = start + size;
 
 		ha->optrom_state = QLA_SREADING;
 		ha->optrom_buffer = vmalloc(ha->optrom_region_size);
@@ -417,8 +418,7 @@ qla2x00_sysfs_write_optrom_ctl(struct file *filp, struct kobject *kobj,
 		}
 
 		ha->optrom_region_start = start;
-		ha->optrom_region_size = start + size > ha->optrom_size ?
-		    ha->optrom_size - start : size;
+		ha->optrom_region_size = start + size;
 
 		ha->optrom_state = QLA_SWRITING;
 		ha->optrom_buffer = vmalloc(ha->optrom_region_size);