[PATCH] dm mpath: fix buffer overflow in parse_hw_handler()

[shao.mingyin]

parse_hw_handler() builds the hw_handler parameter string in a buffer
sized with

	len = 4;

plus one byte per argument.  Those four bytes were meant for the decimal
form of the parameter count, three digits plus the terminating NUL.
dm_read_arg_group() accepts up to 1024 arguments and the count written is
'hw_argc - 1', so a count of 1000 or more needs four digits plus the NUL
and sprintf() writes one byte past the allocation.

Size the buffer from the actual string length instead.

Fixes: 2bfd2e1337f0 ("[SCSI] scsi_dh: Use scsi_dh_set_params() in multipath.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Shao Mingyin <shao.mingyin@xxxxxxxxxx>
---
diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index ae9318f5f21b..2f871a23a8cb 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -1085,8 +1085,9 @@ static int parse_hw_handler(struct dm_arg_set *as, struct multipath *m)
 
 	if (hw_argc > 1) {
 		char *p;
-		int i, j, len = 4;
+		int i, j, len;
 
+		len = snprintf(NULL, 0, "%d", hw_argc - 1) + 1;
 		for (i = 0; i <= hw_argc - 2; i++)
 			len += strlen(as->argv[i]) + 1;
 		p = m->hw_handler_params = kzalloc(len, GFP_KERNEL);