hugetlb, userfaultfd: fix reservation restore on userfaultfd error

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().  Rename new_pagecache_page to
page_in_pagecache to make that clear.

Link: https://lkml.kernel.org/r/20211117193825.378528-1-almasrymina@google.com
Fixes: c7b1850d ("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>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Wei Xu <weixugc@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -5736,13 +5736,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	int ret = -ENOMEM;
 	struct page *page;
 	int writable;
-	bool new_pagecache_page = false;
+	bool page_in_pagecache = false;
 
 	if (is_continue) {
 		ret = -EFAULT;
 		page = find_lock_page(mapping, idx);
 		if (!page)
 			goto out;
+		page_in_pagecache = true;
 	} else if (!*pagep) {
 		/* If a page already exists, then it's UFFDIO_COPY for
 		 * a non-missing case. Return -EEXIST.
@@ -5830,7 +5831,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 		ret = huge_add_to_page_cache(page, mapping, idx);
 		if (ret)
 			goto out_release_nounlock;
-		new_pagecache_page = true;
+		page_in_pagecache = true;
 	}
 
 	ptl = huge_pte_lockptr(h, dst_mm, dst_pte);
@@ -5894,7 +5895,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	if (vm_shared || is_continue)
 		unlock_page(page);
 out_release_nounlock:
-	if (!new_pagecache_page)
+	if (!page_in_pagecache)
 		restore_reserve_on_error(h, dst_vma, dst_addr, page);
 	put_page(page);
 	goto out;