scsi: target: Fix fortify_panic kernel exception

memcmp() requires the two buffers passed as arguments to be at least
'size' bytes long, otherwise a fortify_panic will trigger.

Use memchr_inv() instead of memcmp() to determine whether the received
payload is zeroed or not.

The bug was found by running a block backstore via LIO.

[  496.212958] Call Trace:
[  496.212960] [c0000007e58e3800] [c000000000cbbefc] fortify_panic+0x24/0x38 (unreliable)
[  496.212965] [c0000007e58e3860] [d00000000f150c28] iblock_execute_write_same+0x3b8/0x3c0 [target_core_iblock]
[  496.212976] [c0000007e58e3910] [d000000006c737d4] __target_execute_cmd+0x54/0x150 [target_core_mod]
[  496.212982] [c0000007e58e3940] [d000000006d32ce4] ibmvscsis_write_pending+0x74/0xe0 [ibmvscsis]
[  496.212991] [c0000007e58e39b0] [d000000006c74fc8] transport_generic_new_cmd+0x318/0x370 [target_core_mod]
[  496.213001] [c0000007e58e3a30] [d000000006c75084] transport_handle_cdb_direct+0x64/0xd0 [target_core_mod]
[  496.213011] [c0000007e58e3aa0] [d000000006c75298] target_submit_cmd_map_sgls+0x1a8/0x320 [target_core_mod]
[  496.213021] [c0000007e58e3b30] [d000000006c75458] target_submit_cmd+0x48/0x60 [target_core_mod]
[  496.213026] [c0000007e58e3bd0] [d000000006d34c20] ibmvscsis_scheduler+0x370/0x600 [ibmvscsis]
[  496.213031] [c0000007e58e3c90] [c00000000013135c] process_one_work+0x1ec/0x580
[  496.213035] [c0000007e58e3d20] [c000000000131798] worker_thread+0xa8/0x600
[  496.213039] [c0000007e58e3dc0] [c00000000013a468] kthread+0x168/0x1b0
[  496.213044] [c0000007e58e3e30] [c00000000000b528] ret_from_kernel_thread+0x5c/0xb4

[mkp: tweaked commit message]

Fixes: 2237498f0b5c ("target/iblock: Convert WRITE_SAME to blkdev_issue_zeroout")
Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Reviewed-by: Steven Royer <seroyer@linux.vnet.ibm.com>
Tested-by: Taylor Jakobson <tjakobs@us.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Nicholas Bellinger <nab@linux-iscsi.org>
Cc: <stable@vger.kernel.org> # v4.13+
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_iblock.c

@@ -427,8 +427,8 @@ iblock_execute_zero_out(struct block_device *bdev, struct se_cmd *cmd)
 {
 	struct se_device *dev = cmd->se_dev;
 	struct scatterlist *sg = &cmd->t_data_sg[0];
-	unsigned char *buf, zero = 0x00, *p = &zero;
-	int rc, ret;
+	unsigned char *buf, *not_zero;
+	int ret;
 
 	buf = kmap(sg_page(sg)) + sg->offset;
 	if (!buf)
@@ -437,10 +437,10 @@ iblock_execute_zero_out(struct block_device *bdev, struct se_cmd *cmd)
 	 * Fall back to block_execute_write_same() slow-path if
 	 * incoming WRITE_SAME payload does not contain zeros.
 	 */
-	rc = memcmp(buf, p, cmd->data_length);
+	not_zero = memchr_inv(buf, 0x00, cmd->data_length);
 	kunmap(sg_page(sg));
 
-	if (rc)
+	if (not_zero)
 		return TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
 
 	ret = blkdev_issue_zeroout(bdev,