[PATCH v1] hugetlb, userfaultfd: Fix reservation restore on userfaultfd error

[Mina Almasry <almasrymina@google.com>]

Currently in the is_continue case in hugetlb_mcopy_atomic_pte(), if we
bail out using "goto out_release_unlock;" in the cases where idx >=
size, or !huge_pte_none(), the code will detect that new_pagecache_page
== false, and so call restore_reserve_on_error().
In this case I see restore_reserve_on_error() delete the reservation,
and the following call to remove_inode_hugepages() will increment
h->resv_hugepages causing a 100% reproducible leak.

We should treat the is_continue case similar to adding a page into the
pagecache and set new_pagecache_page to true, to indicate that there is
no reservation to restore on the error path, and we need not call
restore_reserve_on_error().

Cc: Wei Xu <weixugc@google.com>

Fixes: c7b1850dfb41 ("hugetlb: don't pass page cache pages to restore_reserve_on_error")
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reported-by: James Houghton <jthoughton@google.com>
---
 mm/hugetlb.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e09159c957e3..25a7a3d84607 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5741,6 +5741,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 		page = find_lock_page(mapping, idx);
 		if (!page)
 			goto out;
+		/*
+		 * Set new_pagecache_page to true, as we've added a page to the
+		 * pagecache, but userfaultfd hasn't set up a mapping for this
+		 * page yet. If we bail out before setting up the mapping, we
+		 * want to indicate to restore_reserve_on_error() that we've
+		 * added the page to the page cache.
+		 */
+		new_pagecache_page = true;
 	} else if (!*pagep) {
 		/* If a page already exists, then it's UFFDIO_COPY for
 		 * a non-missing case. Return -EEXIST.
--
2.34.0.rc1.387.gb447b232ab-goog