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.com
Signed-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>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9b7fa72fa8fdd49460407664fbf65bec3c9986b9..e8d39b0fe4bd1b7c9375ddbba29e23dabf8a845d 100644 (file)
--- 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);