[PATCH] hugetlb: fix subpool release race

[Yichong Chen]

unlock_or_release_subpool() drops spool->lock before checking whether the
subpool can be freed.  However, subpool_is_free() reads fields that are
updated under spool->lock, including count, used_hpages and rsv_hpages.

Another thread can update those fields before the first thread evaluates
subpool_is_free(), allowing both threads to observe the final freeable
state and release the subpool.

Make the free decision while still holding spool->lock.  Keep the actual
hugetlb_acct_memory() and kfree() calls after dropping the lock.

Fixes: 90481622d757 ("hugepages: fix use after free bug in "quota" handling")
Signed-off-by: Yichong Chen <chenyichong@xxxxxxxxxxxxx>
---
 mm/hugetlb.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e319c6a00555..46ab702c0fc0 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -140,12 +140,14 @@ static inline bool subpool_is_free(struct hugepage_subpool *spool)
 static inline void unlock_or_release_subpool(struct hugepage_subpool *spool,
 						unsigned long irq_flags)
 {
-	spin_unlock_irqrestore(&spool->lock, irq_flags);
+	bool free_subpool = subpool_is_free(spool);
 
 	/* 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 (subpool_is_free(spool)) {
+	spin_unlock_irqrestore(&spool->lock, irq_flags);
+
+	if (free_subpool) {
 		if (spool->min_hpages != -1)
 			hugetlb_acct_memory(spool->hstate,
 						-spool->min_hpages);
-- 
2.51.0