[PATCH v2] scsi: target: pscsi: Fix possible null-pointer dereference in pscsi_complete_cmd()

[Tuo Li <islituo@gmail.com>]

The return value of transport_kmap_data_sg() is assigned to the variable
buf:
  buf = transport_kmap_data_sg(cmd);

And then it is checked:
  if (!buf) {

This indicates that buf can be NULL. However, it is dereferenced in the
following statements:
  if (!(buf[3] & 0x80))
    buf[3] |= 0x80;
  if (!(buf[2] & 0x80))
    buf[2] |= 0x80;

To fix these possible null-pointer dereferences, dereference buf and call 
transport_kunmap_data_sg() only when buf is not NULL.

Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Tuo Li <islituo@gmail.com>
---
v2:
* Put transport_kunmap_data_sg() into the else-branch of the if (!bug).
  Thank Bodo Stroesser for helpful advice.
---
 drivers/target/target_core_pscsi.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/target/target_core_pscsi.c b/drivers/target/target_core_pscsi.c
index 2629d2ef3970..75ef52f008ff 100644
--- a/drivers/target/target_core_pscsi.c
+++ b/drivers/target/target_core_pscsi.c
@@ -620,17 +620,17 @@ static void pscsi_complete_cmd(struct se_cmd *cmd, u8 scsi_status,
 			buf = transport_kmap_data_sg(cmd);
 			if (!buf) {
 				; /* XXX: TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE */
-			}
-
-			if (cdb[0] == MODE_SENSE_10) {
-				if (!(buf[3] & 0x80))
-					buf[3] |= 0x80;
 			} else {
-				if (!(buf[2] & 0x80))
-					buf[2] |= 0x80;
-			}
+				if (cdb[0] == MODE_SENSE_10) {
+					if (!(buf[3] & 0x80))
+						buf[3] |= 0x80;
+				} else {
+					if (!(buf[2] & 0x80))
+						buf[2] |= 0x80;
+				}
 
-			transport_kunmap_data_sg(cmd);
+				transport_kunmap_data_sg(cmd);
+			}
 		}
 	}
 after_mode_sense:
-- 
2.25.1