[PATCH] mm/hugetlb: Fix use after free when subpool max_hpages accounting is not enabled

[Miaohe Lin <linmiaohe@huawei.com>]

When subpool max_hpages accounting is not enabled, 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.

Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/hugetlb.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 777bc0e45bf3..53ea65d1c5ab 100644
--- a/mm/hugetlb.c
+++ b/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);
-- 
2.19.1