hugetlb: remove use of list iterator variable after loop

In preparation to limit the scope of the list iterator to the list
traversal loop, use a dedicated pointer to iterate through the list [1].

Before hugetlb_resv_map_add() was expecting a file_region struct, but in
case the list iterator in add_reservation_in_range() did not exit early,
the variable passed in, is not actually a valid structure.

In such a case 'rg' is computed on the head element of the list and
represents an out-of-bounds pointer.  This still remains safe *iff* you
only use the link member (as it is done in hugetlb_resv_map_add()).

To avoid the type-confusion altogether and limit the list iterator to the
loop, only a list_head pointer is kept to pass to hugetlb_resv_map_add().

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Link: https://lkml.kernel.org/r/20220331224323.903842-1-jakobkoschel@gmail.comSigned-off-by: Jakob Koschel <jakobkoschel@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: "Brian Johannesmeyer" <bjohannesmeyer@gmail.com>
Cc: Cristiano Giuffrida <c.giuffrida@vu.nl>
Cc: "Bos, H.J." <h.j.bos@vu.nl>
Cc: Jakob Koschel <jakobkoschel@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -370,7 +370,7 @@ static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
 }
 
 static inline long
-hugetlb_resv_map_add(struct resv_map *map, struct file_region *rg, long from,
+hugetlb_resv_map_add(struct resv_map *map, struct list_head *rg, long from,
 		     long to, struct hstate *h, struct hugetlb_cgroup *cg,
 		     long *regions_needed)
 {
@@ -379,7 +379,7 @@ hugetlb_resv_map_add(struct resv_map *map, struct file_region *rg, long from,
 	if (!regions_needed) {
 		nrg = get_file_region_entry_from_cache(map, from, to);
 		record_hugetlb_cgroup_uncharge_info(cg, h, map, nrg);
-		list_add(&nrg->link, rg->link.prev);
+		list_add(&nrg->link, rg);
 		coalesce_file_region(map, nrg);
 	} else
 		*regions_needed += 1;
@@ -402,47 +402,52 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
 	long add = 0;
 	struct list_head *head = &resv->regions;
 	long last_accounted_offset = f;
-	struct file_region *rg = NULL, *trg = NULL;
+	struct file_region *iter, *trg = NULL;
+	struct list_head *rg = NULL;
 
 	if (regions_needed)
 		*regions_needed = 0;
 
 	/* In this loop, we essentially handle an entry for the range
-	 * [last_accounted_offset, rg->from), at every iteration, with some
+	 * [last_accounted_offset, iter->from), at every iteration, with some
 	 * bounds checking.
 	 */
-	list_for_each_entry_safe(rg, trg, head, link) {
+	list_for_each_entry_safe(iter, trg, head, link) {
 		/* Skip irrelevant regions that start before our range. */
-		if (rg->from < f) {
+		if (iter->from < f) {
 			/* If this region ends after the last accounted offset,
 			 * then we need to update last_accounted_offset.
 			 */
-			if (rg->to > last_accounted_offset)
-				last_accounted_offset = rg->to;
+			if (iter->to > last_accounted_offset)
+				last_accounted_offset = iter->to;
 			continue;
 		}
 
 		/* When we find a region that starts beyond our range, we've
 		 * finished.
 		 */
-		if (rg->from >= t)
+		if (iter->from >= t) {
+			rg = iter->link.prev;
 			break;
+		}
 
-		/* Add an entry for last_accounted_offset -> rg->from, and
+		/* Add an entry for last_accounted_offset -> iter->from, and
 		 * update last_accounted_offset.
 		 */
-		if (rg->from > last_accounted_offset)
-			add += hugetlb_resv_map_add(resv, rg,
+		if (iter->from > last_accounted_offset)
+			add += hugetlb_resv_map_add(resv, iter->link.prev,
 						    last_accounted_offset,
-						    rg->from, h, h_cg,
+						    iter->from, h, h_cg,
 						    regions_needed);
 
-		last_accounted_offset = rg->to;
+		last_accounted_offset = iter->to;
 	}
 
 	/* Handle the case where our range extends beyond
 	 * last_accounted_offset.
 	 */
+	if (!rg)
+		rg = head->prev;
 	if (last_accounted_offset < t)
 		add += hugetlb_resv_map_add(resv, rg, last_accounted_offset,
 					    t, h, h_cg, regions_needed);