mm/hugetlb: fix use after free when subpool max_hpages accounting is not enabled

If a hugetlbfs filesystem is created with the min_size option and
without the size option, used_hpages is always 0 and might lead to
release subpool prematurely because it indicates no pages are used now
while there might be.

In order to fix this issue, we should check used_hpages == 0 iff
max_hpages accounting is enabled.  As max_hpages accounting should be
enabled in most common case, this is not worth a Cc stable.

[mike.kravetz@oracle.com: new changelog]

Link: https://lkml.kernel.org/r/20210126115510.53374-1-linmiaohe@huawei.comSigned-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -97,16 +97,26 @@ static inline void ClearPageHugeFreed(struct page *head)
 /* Forward declaration */
 static int hugetlb_acct_memory(struct hstate *h, long delta);
 
-static inline void unlock_or_release_subpool(struct hugepage_subpool *spool)
+static inline bool subpool_is_free(struct hugepage_subpool *spool)
 {
-	bool free = (spool->count == 0) && (spool->used_hpages == 0);
+	if (spool->count)
+		return false;
+	if (spool->max_hpages != -1)
+		return spool->used_hpages == 0;
+	if (spool->min_hpages != -1)
+		return spool->rsv_hpages == spool->min_hpages;
+
+	return true;
+}
 
+static inline void unlock_or_release_subpool(struct hugepage_subpool *spool)
+{
 	spin_unlock(&spool->lock);
 
 	/* If no pages are used, and no other handles to the subpool
 	 * remain, give up any reservations based on minimum size and
 	 * free the subpool */
-	if (free) {
+	if (subpool_is_free(spool)) {
 		if (spool->min_hpages != -1)
 			hugetlb_acct_memory(spool->hstate,
 						-spool->min_hpages);