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/
Link: https://lkml.kernel.org/r/20220331224323.903842-1-jakobkoschel@gmail.com
Signed-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>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f8e048b939c749163d5cfb5e1b0c9638e9925883..34efd507a68d3d9522c3fcf5937caea2a4052675 100644 (file)
--- a/mm/hugetlb.c
+++ b/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);