[PATCH v2] cifs: small underflow in cnvrtDosUnixTm()

[Dan Carpenter <dan.carpenter-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

January is month 1.  There is no zero-th month.  If someone passes a
zero month then it means we read from one space before the start of the
total_days_of_prev_months[] array.

We may as well also be strict about days as well.

Fixes: 1bd5bbcb6531 ("[CIFS] Legacy time handling for Win9x and OS/2 part 1")
Signed-off-by: Dan Carpenter <dan.carpenter-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
---
v2:  Be strict about days as well.  My first patch was less intrusive
because it only prevented the out of bounds access.  I have no idea how
common it is to pass in an illegal day but, hopefully, not very common.

diff --git a/fs/cifs/netmisc.c b/fs/cifs/netmisc.c
index abae6dd2c6b9..4b2726ee4fad 100644
--- a/fs/cifs/netmisc.c
+++ b/fs/cifs/netmisc.c
@@ -980,10 +980,10 @@ struct timespec cnvrtDosUnixTm(__le16 le_date, __le16 le_time, int offset)
 		cifs_dbg(VFS, "illegal hours %d\n", st->Hours);
 	days = sd->Day;
 	month = sd->Month;
-	if ((days > 31) || (month > 12)) {
+	if (days < 1 || days > 31 || month < 1 || month > 12) {
 		cifs_dbg(VFS, "illegal date, month %d day: %d\n", month, days);
-		if (month > 12)
-			month = 12;
+		days = (days < 1) ? 1 : ((days <= 31) ? days : 31);
+		month = (month < 1) ? 1 : ((month <= 12) ? month : 12);
 	}
 	month -= 1;
 	days += total_days_of_prev_months[month];