scsi: target: tcmu: Fix crash on ARM during cmd completion

If tcmu_handle_completions() has to process a padding shorter than
sizeof(struct tcmu_cmd_entry), the current call to
tcmu_flush_dcache_range() with sizeof(struct tcmu_cmd_entry) as length
param is wrong and causes crashes on e.g. ARM, because
tcmu_flush_dcache_range() in this case calls
flush_dcache_page(vmalloc_to_page(start)); with start being an invalid
address above the end of the vmalloc'ed area.

The fix is to use the minimum of remaining ring space and sizeof(struct
tcmu_cmd_entry) as the length param.

The patch was tested on kernel 4.19.118.

See https://bugzilla.kernel.org/show_bug.cgi?id=208045#c10

Link: https://lore.kernel.org/r/20200629093756.8947-1-bstroesser@ts.fujitsu.com
Tested-by: JiangYu <lnsyyj@hotmail.com>
Acked-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: BStroesser

drivers/target/target_core_user.c

@@ -1221,7 +1221,14 @@ static unsigned int tcmu_handle_completions(struct tcmu_dev *udev)
 
 		struct tcmu_cmd_entry *entry = (void *) mb + CMDR_OFF + udev->cmdr_last_cleaned;
 
-		tcmu_flush_dcache_range(entry, sizeof(*entry));
+		/*
+		 * Flush max. up to end of cmd ring since current entry might
+		 * be a padding that is shorter than sizeof(*entry)
+		 */
+		size_t ring_left = head_to_end(udev->cmdr_last_cleaned,
+					       udev->cmdr_size);
+		tcmu_flush_dcache_range(entry, ring_left < sizeof(*entry) ?
+					ring_left : sizeof(*entry));
 
 		if (tcmu_hdr_get_op(entry->hdr.len_op) == TCMU_OP_PAD) {
 			UPDATE_HEAD(udev->cmdr_last_cleaned,