fs/ntfs3: Validate ff offset

This adds sanity checks for ff offset. There is a check
on rt->first_free at first, but walking through by ff
without any check. If the second ff is a large offset.
We may encounter an out-of-bound read.
Signed-off-by: lei lu <llfamsec@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/fslog.c

@@ -724,7 +724,8 @@ static bool check_rstbl(const struct RESTART_TABLE *rt, size_t bytes)
 
 	if (!rsize || rsize > bytes ||
 	    rsize + sizeof(struct RESTART_TABLE) > bytes || bytes < ts ||
-	    le16_to_cpu(rt->total) > ne || ff > ts || lf > ts ||
+	    le16_to_cpu(rt->total) > ne ||
+			ff > ts - sizeof(__le32) || lf > ts - sizeof(__le32) ||
 	    (ff && ff < sizeof(struct RESTART_TABLE)) ||
 	    (lf && lf < sizeof(struct RESTART_TABLE))) {
 		return false;
@@ -754,6 +755,9 @@ static bool check_rstbl(const struct RESTART_TABLE *rt, size_t bytes)
 			return false;
 
 		off = le32_to_cpu(*(__le32 *)Add2Ptr(rt, off));
+
+		if (off > ts - sizeof(__le32))
+			return false;
 	}
 
 	return true;